key: cord-0687451-o8zr4f1h
authors: Chakraborty, Saborni; Mallajosyula, Vamsee; Tato, Cristina M.; Tan, Gene S.; Wang, Taia T.
title: SARS-CoV-2 vaccines in advanced clinical trials: where do we stand
date: 2021-01-20
journal: Adv Drug Deliv Rev
DOI: 10.1016/j.addr.2021.01.014
sha: 4079d307520a6d6b49062c61a906a940c52a8c30
doc_id: 687451
cord_uid: o8zr4f1h

The ongoing SARS-CoV-2 pandemic has led to the focused application of resources and scientific expertise toward the goal of developing investigational vaccines to prevent COVID-19. The highly collaborative global efforts by private industry, governments and non-governmental organizations have resulted in a number of SARS-CoV-2 vaccine candidates moving to Phase III trials in a period of only months since the start of the pandemic. In this review, we provide an overview of the preclinical and clinical data on SARS-CoV-2 vaccines that are currently in Phase III clinical trials and in few cases authorized for emergency use. We further discuss relevant vaccine platforms and provide a discussion of SARS-CoV-2 antigens that may be targeted to increase the breadth and durability of vaccine responses.

The current international efforts toward quelling the ongoing SARS-CoV-2 pandemic are unprecedented in pace, resource allocation and scientific focus. In less than one year since the first cases of COVID-19 were identified, vaccines showing early signs of promise are already in the later phases of clinical testing and in some cases are approved for emergency use. The accessory proteins have been shown to influence the pathogenicity and virulence of the virus by modulating the cytokine response, Type I interferon signaling pathways, and cellular apoptosis [36, 37] . As our understanding of SARS-CoV-2 evolvesspecifically concerning the nomenclature of orthologous accessory genes and overlapping genes discovered within ORFs [38] we anticipate that new genes and proteins may yet be discovered. Of note, accessory

proteins encoded by open reading frame (ORF) 3a [39] and ORF7a [40] were found on SARS-CoV-1 viral particles, but have yet to be validated for SARS-CoV-2.

In this section, we have focused on describing the four structural proteins which are the major targets of the adaptive immune response during SARS-CoV-2 infection. In addition, we discuss the accessory proteins ORF3a and ORF7a as there is evidence that these proteins are expressed on infected cell membranes (unlike the structural proteins), potentially making them targets to explore in the context of SARS-CoV-2 vaccines, in addition to the spike protein.

The surface glycoprotein S is responsible for binding to the major host receptor for SARS-CoV-2, angiotensin converting enzyme 2 (ACE2), and for mediating fusion between the viral and host cell membranes. This makes S protein the principal target of neutralizing antibody responses [41] [42] [43] which are so far the best described correlate of protection against infection in humans [44] [45] [46] [47] [48] [49] [50] [51] [52] , and in animal challenge studies [45, [53] [54] [55] [56] [57] . The SARS-CoV-2 S is a class I homotrimeric transmembrane protein that is expressed on the surface of the viral particle and exists in a prefusion metastable conformation. A monomer is composed of two subunits, S1 and S2, that have distinct functions responsible for viral entry. The S1 subunit contains the receptor binding domain (RBD) and allows recognition and binding to ACE2, while the S2 subunit contains the fusion peptide that mediates the fusion of the host and viral membrane (Figure 2c) The membrane (M) and envelope (E) proteins, along with the S protein, are the main structural components forming the shell of the viral particle. While the S is the major determinant of viral entry, the role of M is in viral assembly and budding [79] and the E is a putative ion channel involved in viral replication [80] [81] [82] . The M protein is the most abundant viral protein [83] and, in combination with either the N [84, 85] or E [86] [87] [88] , can drive the budding of virus-like particles.

E protein is largely expressed in the endoplasmic reticulum and Golgi-complex of the host cell and is not highly incorporated in viral particles [89] [90] [91] [92] . Antibody [93] and T cell [94] responses against the M protein have been reported in people infected with SARS-CoV-2. However, the role of M-based immunity in protection is not yet known. The immune response to E has not yet been well characterized and there is currently little available data that highlights its immunogenicity and/or role in immunity.

The N protein is a multi-functional protein that can be divided into an N-terminal RNA binding domain and a C-terminal oligomerization region that are connected by a disordered linker region [95] . The N is involved in many aspects of viral replication including RNA packaging, replication and pathogenesis [96] . Via the RNA binding region, N binds to the viral genome and packages it into a long and helical ribonucleoprotein complex which protects the viral RNA from degradation. The nucleocapsid-RNA interaction in the context of genomic and subgenomic (sg) viral RNA is intimately involved in viral transcription and translation [97, 98] . Multiple interactions between N and host cellular pathways that may contribute to viral pathogenesis have been described. These include disruption of the host translation machinery, cell cycle, inhibition of Type I interferon signaling, induction of apoptosis and modulation of the transforming growth factor beta pathway [83] [84] [85] [86] [87] 99] .

The N of SARS-CoV-1 and -2 is highly immunogenic and is the target of antibody [100] [101] [102] [103] [104] [105] and T cell-mediated responses [106] [107] [108] [109] [110] [111] [112] . Of note, some SARS-CoV-1 studies demonstrated an enhanced respiratory disease (ERD) after SARS-CoV-1 challenge in mice [113] [114] [115] [116] and in ferrets [117] [118] [119] , where the pathogenesis of ERD was thought to be driven by a Th2 response [113] against the N protein [114, 116] . ERD in challenged mice was not reproduced following passive transfer of immune sera that was enriched for N-specific antibodies suggesting that these antibodies may not have been responsible for the ERD phenotype [114] . The role of N antibodies in protection may be limited since it is not thought to be expressed on the surface of viral particles or infected cells. However, the role of N antibody responses in protection or J o u r n a l P r e -p r o o f disease warrants further investigation. A recent study suggested that individuals who succumbed to COVID-19 had a stronger N-specific antibody response than patients with severe disease who ultimately recovered [120] . By contrast, the contribution of T cell-mediated immunity against N may be supported by studies from SARS-CoV-1, SARS-CoV-2 and MERS-CoV [121, 122] . Indeed, a growing body of literature suggests that N is a major target of CD4+ and CD8+ T cells [94, 123] and these responses may correlate with the resolution of COVID-19 in some patients [124] .

Much of what is known about the protein encoded by ORF3a is based on studies performed with SARS-CoV-1. The ORF3a of SARS-CoV-1, also known as ORF3 [125] , X1 [126] , ORF3a [127] and U274 [128] , is located at the 3' end of the genome residing between the S and the E genes and encodes for the largest of the accessory proteins. It is 274 amino acids long and has an extracellular N-terminal ectodomain, three transmembrane domains and a C-terminal cytosolic domain. Putatively characterized as an ion channel [129] or viroporin, ORF3a expression has been detected in the rough endoplasmic reticulum [130] , the Golgi complex [127] and on the plasma membrane [127, 130, 131] of infected cells. Of note, it is also found as extracellular membrane-associated structures secreted by virus-infected cells [132] . The SARS-CoV-2 ORF3a protein is ~72% identical to the ORF3a of SARS-CoV-1 and current literature indicates it largely shares structural features with other betacoronavirus ORF3a proteins [133, 134] . Additional studies suggest a possible role in pathogenesis and virulence [135] [136] [137] [138] [139] [140] .

While ORF3a may be expressed on viral particles at some level, it is not generally associated with viral entry suggesting that anti-ORF3a antibodies will not neutralize virus by preventing binding or fusion. Yet data supporting its immunogenicity and expression on infected cells suggests that it may be an important target for the many antibody and T cell mediated mechanisms other than virus neutralization that can contribute to broad and durable antiviral immunity.

Based on its SARS-CoV-1 orthologue, the ORF7a encodes for an accessory type I transmembrane protein that is 122 amino acid long [141, 142] . The SARS-CoV-2 orthologue (YP_009724395.1) is 121 amino acids long and is ~86% identical to the ORF7a of SARS-CoV-1 (NP_828857.1). Deletional mutant studies demonstrate that the ORF7a protein is not required J o u r n a l P r e -p r o o f for replication in vivo and in vitro [142] [143] [144] . It is, however, involved in the induction of caspasedependent apoptosis [142, 145, 146] . Expression of ORF7a can be found intracellular in the Golgi complex and on the surface of the plasma membrane of infected cells.

The role of ORF7a in virulence and immunity is not entirely known. The finding that ORF7a is on the surface of infected cells makes it a likely target of the antibody response and recent work by several groups has demonstrated antibody responses against SARS-CoV-2 ORF7a [147, 148] .

Others have also found ORF7a to be targeted by T cell responses [94, 149] . The ORF7a region underwent positive selection during the SARS-CoV-1 zoonotic outbreak earlier in the century [150] and more recently, deletional variants in the SARS-CoV-2 pandemic have been found in circulation [151, 152] . Whether or not these mutations occurred due to immunological pressure or natural selection remains to be determined. Nonetheless, ORF7a is immunogenic and is recognized by both arms of the adaptive immune systemwhether or not this response contributes to protection warrants further investigation.

A wide array of vaccine platforms, each with its own advantages and disadvantages, is represented among the experimental vaccines being trialed to prevent COVID-19. Each of these platforms is represented among SARS-CoV-2 vaccine candidates that are in pre-clinical and/or clinical stages of development ( Figure 3 ). The vaccine platforms can be broadly classified as follows (platforms already in use in humans for non-SARS-CoV-vaccines are denoted by *):

Inactivated virus (IV) vaccines are being developed based on SARS-CoV-2 isolates cultured from hospitalized COVID-19 patients. The virus is passaged in Vero cells-a WHO certified cell line for vaccine production and is subsequently chemically inactivated using β-propiolactone chimpanzee adenovirus (ChAd) [175, [181] [182] [183] [184] . As with LAV vaccines, VV-based vaccines are not suitable for use in immunocompromised populations and they can be reactogenic (causing "expected" adverse reactions after vaccination) in healthy people [185, 186] . Finally, there are not yet any licensed VV-based vaccines for use in humans, though Ebola virus and Chikungunya virus vaccines using this platform are in advanced phases of clinical trials; early safety and immunogenicity data for these vaccines are promising [187] [188] [189] [190] [191] . For SARS-CoV-2 VV-based vaccines, the S protein (and stabilized variants) are the most common antigens being trialed.

Based on the property of the viral vector, this category of vaccine platform can be further classified, as follows: a) Non-replicating viral vector (NRVV)

Here, the viral vectors themselves are replication incompetent and their function is to simply deliver the transgene cargo. Four out of ten of the VV-based SARS-CoV-2 vaccines that are in Phase III trials are based on NRVV platforms and all use adenoviral vectors [48, 50, 51, 192] . Other vectors that are in Phase I and preclinical evaluation are gorilla adenovirus, Sendai virus vector, adeno-associated virus vector, modified vaccinia virus Ankara, parainfluenza virus 5, deactivated rabies virus and influenza A virus H1N1 vectors [8, [193] [194] [195] [196] [197] b) Replicating viral vector (RVV) Multiple SARS-CoV-2 vaccines are also being developed with attenuated viral vectors that can replicate at the site of immunization and can thus elicit stronger immune responses using relatively small vaccine doses. There are two SARS-CoV-2 candidates in Phase I/II clinical trials that use measles (NCT04497298) or vesicular stomatitis virus (VSV) (NCT04569786) both developed by Merck Sharp & Dohme [198] . Additionally, there are multiple candidates in preclinical evaluation based on yellow fever, measles virus, horsepox vector, attenuated influenza virus, Newcastle disease virus (NDV) and avian paramyxovirus (APMV) vectors [8, [199] [200] [201] .

Nucleic acid-based platforms utilize delivery of either plasmid DNA or RNA that codes for an immunogen of interest. Following cellular uptake, they are either transcribed and translated (DNA) or directly translated (RNA) to protein antigens that can elicit both humoral and T-cell J o u r n a l P r e -p r o o f mediated immune responses. These vaccines can be extremely useful in a pandemic setting as they are scalable and can be rapidly synthesized without handling infectious virus. There are currently no licensed human vaccines based on these platforms, but a number are in clinical evaluation for influenza virus [202] [203] [204] , Ebola virus [205] , Zika virus [206] and MERS [207] . a) DNA These vaccines are designed by cloning of a eukaryotic protein expression cassette consisting of the gene of interest downstream of a strong promoter into a bacterial plasmid that can be propagated in organisms like Escherichia coli for mass production.

Upon immunization and cellular uptake at immunization sites, the plasmid must traverse the nuclear membrane for transcription, followed by expression of the immunogen in the cell cytoplasm. Because of the complex cellular requirements for protein production, these vaccines may not be as immunogenic as RNA or protein-based vectors and may require specialized delivery techniques like electroporation [176, 208] . There are currently four DNA vaccines under clinical evaluation [8] . Pre-clinical data from one candidate DNA vaccine (INO-4800) developed by Inovio demonstrated immunogenicity in mouse and guinea pig models [209] . In a rhesus macaque model, vaccination with two doses of a DNA-encoding S protein protected against SARS-CoV-2 challenge 3 months after vaccination [210] . b) RNA RNA platforms are either self-amplifying (sa) alphavirus-derived RNA (saRNA) or nonreplicating mRNA (nrmRNA). Current SARS-CoV-2 RNA vaccines encode the stabilized S protein and are encapsulated in lipid nanoparticles for efficient delivery. These comprise some top SARS-CoV-2 candidates in clinical and pre-clinical evaluation, as discussed below [211] [212] [213] . Compared to the DNA vaccine platform, RNAs are more immunogenic as they can be translated into the protein of interest without requiring nuclear entry for transcription (they are translated directly in the cytosol following uptake into cells at the injection site) [214] . RNA vaccines are thought to result in production of natively folded and processed proteins, leading to effective antigen presentation and B cell activation. RNA vaccines do not require electroporation as DNA vaccines can [215, 216] . Potentially serious considerations around RNA vaccines are related to their stability, a requirement for any SARS-CoV-2 vaccine that will be widely administered [217] . Further, large scale safety data in humans are lacking for nucleic acid platforms J o u r n a l P r e -p r o o f Journal Pre-proof [218] . Moderna, the company developing the SARS-CoV-2 nrmRNA vaccine currently in Phase III trials has previously developed two nrmRNA vaccines against Zika virus [219, 220] . Both of these Zika virus vaccines are in Phase I clinical trials following promising data from preclinical animal studies, but no data has yet been published from either trial.

Immunogenic viral proteins expressed in bacterial, insect or mammalian cells generally make effective and safe vaccines and can be administered with an adjuvant to boost the immune response [221, 222] . Another strategy to enhance immunogenicity of subunit protein vaccines is multimeric antigen display and presentation by conjugation to carrier nanoparticles, a platform that is being explored for SARS-CoV-2 vaccine design [223, 224] . Although relative to nucleic acid-based or viral vector-based vaccines, protein subunit vaccines are expensive and laborious to produce, there are already several licensed recombinant protein-based vaccines including for hepatitis B and influenza viruses [156, [225] [226] [227] . There are currently 11 protein subunit SARS-CoV-2 vaccines in Phase I/II clinical trials and one in advanced Phase III trial [8, 228] .

A special category of recombinant protein-based platform, VLPs are made from viral proteins that self-assemble into supramolecular structures that are meant to mimic native virions in geometry and size [229, 230] . Because of this, they are often highly immunogenic and elicit strong innate and adaptive immune responses (both humoral and cell-mediated) [231] . Licensed vaccines against human papillomavirus and hepatitis B viruses use the VLP platform and are widely used in humans [232] . A plant derived VLP vaccine generated by Medicago that is adjuvanted with one of the two different adjuvants (GSK's pandemic adjuvant technology and Dynavax's CpG 1018) is currently in Phase I clinical trials [233] .

Early-phase clinical studies evaluate safety and immunogenicity of vaccine candidates. With respect to immunogenicity, antibody responses are typically studied by measuring serum titers (usually by ELISA) to S or RBD; in this context, some studies specify the antibody isotype being characterized (particularly IgG). Neutralizing antibody titers are generally measured in in vitro assays using a replicating SARS-CoV-2 virus or non-replicating pseudovirus.

In addition to measuring antibodies, SARS-CoV-2 vaccine clinical trials have also carried out preliminary characterization of the cell mediated immune response. Typically, in a subset of randomly selected volunteers, the frequency of T cells (CD3+CD4+ and CD3+CD8+) and cytokine secretion was measured at different time-points, with or without stimulation using overlapping peptide pools from the S protein to evaluate Th1 polarization, which is considered to be an important marker for vaccine selection. Vaccine studies that use whole virus-based vaccine platforms may evaluate antibody and T cell responses against non-S proteins of SARS-CoV-2, but most studies focus on S protein as this is the sole antigen in nearly all vaccines being tested in Phase III studies.

Vaccine target: whole virus A SARS-CoV-2 isolate (19nCoV-CDC-Tan-HB02 (HB02)) from a hospitalized COVID-19 patient was used to develop this vaccine. The HB02 isolate was passaged seven times in Vero cells to generate the vaccine stock which was then inactivated with β-propionolactone. Electron microscopy and western blot analysis of the inactivated virus were performed to characterize the integrity of the viral particles and their surface antigens following inactivation [153] .

Immunogenicity of BBIBP-CorV was first assessed in BALB/c mice. Animals were immunized intraperitoneally with 2, 4, or 8μg of the vaccine which was adjuvanted with aluminum hydroxide The nAb titers increased until day 21 in the low and middle dose group but did not change in the high dose group after day 14. In the two-dose immunization group, where different time points between prime and boost regimen were tested, the D0/D21 regimen elicited the highest nAb level on day 7 post-boost. Immunogenicity of the two-dose regimen was significantly enhanced over the one-dose schedule. Immunogenicity of the three-dose (D0/D7/D14) regimen was highest among all regimens, at all dose levels. Immunogenicity was also measured in additional animal models: rabbits, guinea pigs, rats, and cynomolgus monkeys. In all models, 100% of animals had produced nAbs on day 21 with higher titers elicited by three immunizations compared to one [153] .

Immunogenicity and protection were then evaluated in rhesus macaques. All macaques were immunized twice on D0 and D14 with either placebo (saline), or with a "low" dose (2μg/dose) or "high" dose (8μg/dose) of BBIBP-CorV. The Geometric mean titers (GMTs) of nAb in the lowdose and high-dose groups reached 215 and 256, respectively at D24, the day of challenge.

Animals were challenged with 10 6 TCID 50 of SARS-CoV-2. All macaques in the placebo group showed sustained high viral load up to 7 days after virus challenge from both throat and anal swab samples. In contrast, the viral load in the throat swabs of the low dose group peaked at 5 days and decreased by day 7 post-infection to a level that was significantly lower than that of the placebo group. Three out of four macaques in the low-dose group had no detectable viral load at 7 days post-infection. In the high dose group, all four macaques had undetectable viral load in their throat swabs. Moreover, no viral load was detected in the anal swabs of 2 out of 4 macaques. At 7 days post-infection, lung tissues from euthanized animals were examined for pathology and viral load. No macaques in the low-dose or high-dose groups had detectable lung viral loads and lung histology was largely normal. In comparison, the placebo group showed severe interstitial pneumonia and high lung viral loads [153] . J o u r n a l P r e -p r o o f Immunization Schedule: prime-boost (days 0 and 28), multiple dosages tested, intramuscular administration A prime-boost regimen was tested in which doses were 2, 4, or 8μg BBIBP-CorV adjuvanted with aluminum hydroxide (n=24 per dose group). The placebo group received saline plus adjuvant. For vaccine recipients aged 18-59 years, 79% to 96% seroconverted, defined by nAb production, by day 14 (with a dose-dependent increase in nAb titers). 100% of vaccine recipients produced nAbs by day 28. The vaccine was somewhat less immunogenic in the older cohorts and seroconversion was delayed. Seroconversion rates in those ≥60 year of age were 4% in the 2μg group and 46% for the 4μg and 8μg groups on day 14 post-boost but increased to 91% to 96% in the three dose groups by day 28. NAb GMTs in the 2μg cohorts were significantly lower than the 8μg cohorts, irrespective of age. In 6 randomly selected participants from the 4μg dose group, sera collected on day 42 had neutralizing activity against ten different natural SARS-CoV-2 isolates, four of which contained the well-described D614G spike mutation [234] .

All adverse reactions (ARs) were mild or moderate in severity with 29% of vaccine recipients reporting at least one AR within the first 7 days of vaccination. No serious adverse events (AEs) were reported within the 28 days after vaccination [47] Phase II: Self-limiting AEs of mild to moderate severity were reported with no report of severe AEs. 23% of vaccine recipients reported at least one AR within the first 7 days after either vaccination. The reactogenicity profile was similar to the Phase I study where pain at the site of injection was the most common local AE, with significantly higher number in the vaccine group as compared to the placebo. Most common systemic adverse reaction in the vaccine recipient group was fever reported in 2% of the vaccine group [47] . In the Phase I trial, participants were assigned to one of three vaccine dose groups of 2.5, 5, or 10μg per dose. Three vaccinations of each dose (or placebo) were given on days 0, 28, and 56.

In the Phase II trial, participants received a prime-boost consisting of 5μg per dose (or placebo).

The boost was administered on day 14 or 21. The placebo consisted of alum adjuvant only.

In the Phase I study, 100% of participants in the low and high dose groups seroconverted whereas 96% in the medium dose group seroconverted 14 days after the third dose. In the Phase II study, 97.6% seroconverted 14 days post boost regardless of the interval between prime and boost vaccinations. The nAb GMT was 2-fold higher in the group who received injections on days 0/21 as compared to the group who received injections on days 0/14.

Because prior studies have reported that using alum as an adjuvant can bias the vaccine response towards a Th2 phenotype which can lead to VAERD [235, 236] , an attempt was made to assess whether alum adjuvantation of the Wuhan Institute of Biological Products/Sinopharm vaccine had any such effects. Blood lymphocyte subsets (NK, CD4+ T, CD8+ T and B cells) and serum cytokines were measured in the peripheral blood of participants in Phase I trials at day 0 and 14 days after each injection. An extensive panel of serum cytokines (IL-1, IL-2, IL-4, IL-5, IL-6, IL-8, IL-10, IL-12P70, IL-17A, IL-17F, IFN-, TNF-, TNF-) was measured. There were no notable changes between the vaccine or alum-only groups over time, with respect to lymphocyte subsets or the measured cytokines (including the Th2 related cytokines IL-4, IL-5 and IL-10) [154] . T-cell mediated immune responses after antigen stimulation were not measured.

The inactivated vaccine was safe in all dose groups and vaccination schedules. All adverse reactions were mild (grade 1 or 2), transient, and self-limiting, and did not require any treatment [154] .

Placebo controlled: yes Participants: healthy adults, 18 years of age (n=thousands between multiple trials) Immunization schedule: intramuscular administration.

There are currently three international clinical trials of the vaccine underway. In Peru, 6,000 healthy adults 18 years of age are being enrolled (NCT04612972). In the United Arab Emirates (Abu Dhabi) 15,000 healthy adults aged 18 years are being enrolled (ChiCTR2000034780) where participants will either receive BBIBP-CorV or this vaccine or placebo (n=5000 each group). In both of these studies, the primary outcome measured is "protective effect against COVID-19, after 14 days following the full course of vaccination". A third study is being conducted in Morocco, consisting of 600 healthy adults aged 18 years old (ChiCTR2000039000). In Morocco, the primary goal is "to evaluate the 4-fold increase rate, GMT and GMI of anti-SARS-CoV-2 neutralizing antibody 28 days after full course of immunization".

Vaccine: CoronaVac (previously known as PiCoVacc)

This vaccine is based on a SARS-CoV-2 isolate (CN2) from a hospitalized COVID-19 patient which was then adapted for efficient growth in Vero cells. Genetic stability of the strain was ascertained by passaging 10 times in culture followed by whole genome sequencing. The virus was inactivated using β-propiolactone. Following inactivation, the structural and antigenic integrity of viral particles was assessed by cryo-electron microscopy which showed intact, crown-like spikes of the prefusion S protein decorating the viral surface [155] .

Preclinical data Immunogenicity was tested in BALB/c mice immunized at days 0 and 7 with various doses of CoronaVac mixed with alum adjuvant (0, 1.5, 3, or 6μg per dose). No inflammation or other vaccine associated adverse effects were observed in the vaccinated animals. Anti-S, RBD, and N antibody responses were evaluated by ELISA. Rapid elicitation of S-specific and RBD-specific IgG titers occurred, peaking at week six post-vaccination. 50% of the anti-S IgG were directed against RBD, establishing it's immunodominance. The vaccine did not induce a robust anti-N response, with ~30-fold lower titers at week 6 as compared to RBD and S. When benchmarked against sera from convalescent COVID-19 patients, immunized mice had 10-fold higher anti-S and RBD titers at week 6 across all dose groups. NAbs showed a dose dependent increase in titers which peaked at week 6. Sera from immunized mice exhibited broad nAb titers against a panel of 9 viral strains which are circulating globally. CoronaVac had a similar immunogenicity profile in Wistar rats [155] .

The immunogenicity and protective efficacy of CoronaVac was also evaluated in rhesus macaques. Animals were immunized three times intramuscularly with a "medium" dose (3μg per dose) or "high" dose (6μg per dose) of alum-adjuvanted CoronaVac on days 0, 7, and 14 (n = 4). In both the dose groups, S-specific IgG and nAb titers were similar to those from recovered COVID-19 patients by week 3 post-vaccination. At week 3, immunized animals were challenged with 10 6 TCID 50 of SARS-CoV-2. After challenge, all control macaques (which received either adjuvant alone or saline) had high copies of viral genomic RNA in the airways and lung tissue, along with severe interstitial pneumonia. Viral loads were significantly lower in all vaccinated animals with no detectable virus in pharynx, crissum, or lungs on day 7 post-challenge [155] . post-prime. Doses were either 3 or 6μg protein, adjuvanted in alum. Seroconversion rates (measured by serum nAb activity) in those boosted on day 14 were 46% (3ug dose) or 50% (6ug dose). In those boosted on day 28, seroconversion was observed in 83% (3ug dose) or 79% (6ug dose). RBD-specific IgG was detected in 100% of participants only in the 6μg dose in those boosted at day 14, whereas 100% participants in both dose groups seroconverted if they were boosted on day 28. Serum inflammatory factors (IL-1, IL-6 and TNF-) were measured in the blood and urine samples collected 7d after each dose using sandwich ELISA, and no significant difference was observed between the vaccine and placebo groups. T cell responses were evaluated in samples by measuring IFN-collected at different time-points after dose 1 of the vaccine or placebo. The average IFN- spot forming cells (SFCs) per 10 5 cells was highest for the 3g group in both the vaccination cohorts (d0-d14 and d0-d28) in comparison to the 6g and placebo groups.

Low reactogenicity was observed with most of the reported AEs being mild and resolving after day two post-vaccination. One case of an acute hypersensitivity reaction (urticaria) was reported in a 6μg dose group after primary vaccination; this event was considered to be vaccine associated and graded as a severe AE. The participant remained in the study and did not report a similar reaction after the booster dose. Overall, there was no significant difference in AEs among study groups [237] .

Phase II Placebo controlled: yes Participants: healthy adults aged 18-59 years old (n=600) Immunization schedule: prime-boost (same as the Phase I study -variable intervals, variable doses), intramuscular administration.

This was a larger study performed with identical vaccine regimens and doses as in the Phase I trial. Seroconversion rates (measured by serum nAb activity) in those boosted on day 14 were 92% (3ug dose) or 98% (6ug dose). In participants boosted on day 28, seroconversion was observed in 97% (3ug dose) or 100% (6ug dose). When benchmarked against convalescent sera from recovered COVID-19 patients, nAb titers after the second dose in sera of vaccine recipients was lower in all participants. When data from Phase I and Phase II studies were J o u r n a l P r e -p r o o f combined, the correlation coefficient between the nAb titer and RBD-specific IgG titer 28 days post vaccination was 0.85. Higher nAb and binding IgG titers in the Phase II trial (compared to Phase I) was due to a different vaccine production process for the Phase II study which inadvertently increased the content of spike protein in the vaccine used for Phase II studies by almost two-fold. T cell responses were determined by measuring IFN- in an ELISpot assay wherein PBMCs were cultured with overlapping peptides from the S protein or controls peptides [237] .

The reactogenicity profile was similar to the Phase I with most AEs being mild in severity and no reported serious adverse events in the 28 days post-boost vaccination. Considering the combined safety and immunogenicity data, the 3ug dose of CoronaVac was suggested for Phase III trials [237] .

Placebo controlled: yes Participants: multiple trials Immunization schedule: prime-boost regimen 14 days apart, intramuscular administration.

CoronaVac has obtained an emergency approval for use in China, and three Phase III clinical trials there are ongoing. In Brazil, ~13,000 health care workers, aged 18 years and older are being enrolled (ClinicalTrials.gov NCT04456595). In Indonesia, enrollment consists of ~1620 healthy adults aged 18-59 years (ClinicalTrials.gov NCT04508075). Finally, in Turkey, a total of ~13,000 adults aged 18-59 years are being stratified into two separate cohorts, 1300 healthcare workers and 11,150 "people at normal risk" (ClinicalTrials.gov NCT04582344). The primary outcome of protection rate against PCR-confirmed COVID-19 starting 2 weeks after the booster dose is being followed in all current Phase III trials. In the Brazil cohort, a primary outcome of frequency of ARs in 7 days post vaccination is also being measured. Safety was assessed in 108 participants with a median age of 37 and no participants >60 years old. Three dose regimens were tested. A single dose regimen consisting of "low" (5 × 10 10 particles) or "medium" (1 × 10 11 particles) doses, and a third prime-boost regimen with a "high" (1·5 × 10 11 particles) vaccine dose were trialed. Approximately 87% of the participants reported at least one AE within 7 days of vaccination. Overall, >95% of the participants seroconverted to RBD at day 28 post vaccination as measured by ELISA, with those in the high dose group exhibiting the highest titer of nAb responses against live SARS-CoV-2 (GMT 12.7). The greatest number of participants reported grade 3 AEs in the high dose group (17%) as compared to the low and medium dose groups (6% for each). The higher immunogenicity in the high dose group was judged to be offset by higher reactogenicity. Low and medium doses were thus selected for continued testing in a Phase II efficacy trial [52] . Antigen-specific T cell responses were quantified in d14 and d28 post-vaccination samples by measuring IFN- using an ELISpot assay by stimulating fresh PBMCs with overlapping S protein peptide pools for 12-24h before detection. Vaccine induced CD4+ and CD8+ T cell responses were also determined by measuring IFN-, IL-2, and TNF- by ICS after a 6h S protein peptide pool stimulation. The T cell response peaked at d14 post-vaccination, with a slight decrease by d28 across all dosage groups. The proportion of positive responders across all the dosage groups was estimated to range between 83-97% with a dose-dependent increase. Also, the frequency of polyfunctional phenotypes observed in the memory CD4+ T cell subset was higher than those from CD8+ T cells. Immunization schedule: single administration, multiple doses tested, intramuscular administration.

Efficacy was evaluated in 508 participants, of whom 13% were >55. In this study, a single injection of either "low" dose 5 × 10 10 particles or "high" dose 1 × 10 11 particles (equal to the "medium" dose in Phase I) was administered. The rate of seroconversion 28 days post vaccination was 49% with a nAb GMT of 18.3 in the low-dose group, and 59% with a nAb GMT of 19.5 against wild-type SARS-CoV-2 in the high-dose group. Approximately 90% of participants in both dosage groups also showed SARS-CoV-2 spike glycoprotein-specific IFNγ-ELISpot responses at day 28 with median values of 10-11 spot-forming cells per 10 5 peripheral blood mononuclear cells (PBMCs), corresponding to 10-fold increases over baseline prevaccination levels. Sex and age of the participants did not impact the T cell responses induced by vaccination. However, age and pre-existing Ad-5 immunity were inversely correlated with the degree of antigen specific immune responses, particularly seroconversion, raising the concern that a single dose of the vaccine may not be adequate in the elderly or in people with high levels of pre-existing Ad5 immunity. The low dose was selected for Phase III trial.

Both doses were reactogenic with most common AEs reported within 14 days of immunization being fever, fatigue and pain at the site of injection in >50% of individuals. 9% of AEs reported by participants receiving high dose of 1 × 10 11 viral particles had severe (grade 3) adverse reactions, which was significantly higher than the participants receiving the lower dose.

Encouragingly, the grade 3 AEs resolved without any medical intervention within 3-4 days of vaccination. None of the study participants reported any serious AEs within 28 days [192] . Immunization schedule: prime only or prime-boost, multiple doses tested, intramuscular administration Interim data on safety, reactogenicity and immunogenicity has been published. Participants received either 5x10 10 (low dose) or 1x10 11 (high dose) Ad26 particles or placebo. In this ongoing trial, a subset of participants is scheduled to receive a second vaccine dose equivalent to the first. This boost will occur 8 weeks after the primary vaccination. Immunogenicity data is available for all participants 18-55 years old, but only for 15 participants >65 years of age. Seroconversion rates measured by S binding antibody titers day 29 post vaccination was 99% for cohort 1a participants for both dose levels with no significant difference in GMTs.

Seroconversion rate for cohort 3 was 100% with GMTs of 507 and 248 for the low and the high doses respectively. Neutralizing antibody titers were also assessed at day 29 in a subset of participants (n=50 per dose group for those <55 years old and n=6 per dose group for >65

year old). In both dosing regimens, 92% of participants <55 years old developed nAbs, with nAb GMTs between 214-243, while for the >65-year subset, 100% or 83% developed nAbs at the low, or high dose, respectively. In addition to a higher rate of nAb generation, the lower dose elicited higher nAb levels in subjects >65 years of age, with Though some reactogenicity was observed across groups, higher reactogenicity was observed in subjects who were 18-55 and who were given the higher vaccine dose, with 72% of year old and 46% >65year old reporting AEs [50] . 

Platform: replication-deficient simian adenovirus vector ChAdOx1

Vaccine target: full-length, codon-optimized S protein AZD1222 vaccine candidate has full-length codon-optimized S with a tissue plasminogen activator leader sequence cloned in the replication-deficient simian adenovirus vector ChAdOx1.

Preclinical data:

Immunogenicity studies of AZD1222 were performed in mice and in two large animal models-1) pigs and 2) rhesus macaques, and vaccine efficacy was reported from a virus challenge study in Though the study participants were initially planned to receive a single dose of AZD1222, the protocol was subsequently modified to a two-dose prime-boost regimen for the Phase II cohorts due to generation of a more robust immune response in the 10 participants who received a boost in the Phase I trial. Interim analysis of efficacy and safety has been reported using pooled data from four trials-COV001 (Phase I/II), COV002 (Phase II/III), COV003 (Phase III) and COV005 (Phase I/II) [242] .

Data from all four trials were used for assessing safety, whereas efficacy analysis was limited to data from a subset of participants of COV002 and COV003. The vaccine was safe and number of recorded AEs in participants were similar in the placebo and the vaccine groups. There were 3 reported cases of transverse myelitis, two in the vaccine group and one in the placebo group.

Only one of them was judged to be vaccine related and had caused a temporary pause in the study.

Though efficacy was evaluated by pooling data from the COV002 and COV003 trials, there were differences in vaccination regimen and doses within cohorts of the COV002 trial and also between the two trials. For the UK COV002 study, participants aged 18-55 were recruited first followed by phased recruitments of older cohorts (56-69 years and >70 years). Firstly, due to an error in the method used to quantify the number of viral particles in the vaccine during the early stages of the trial, a subset of participants in the 18-55 age cohort (n=1367) received half (2.2 × 10 10 viral particles) of the intended standard dose (SD) (5 × 10 10 viral particles) for the prime. Secondly, the study was initially intended to be a single dose efficacy study but was later amended to be a two dose regimen based on Phase I results. So, there was considerable lag between the prime and boost (>12 weeks, median gap of 84 days) especially for the participants in the 18-55 age cohort, many of whom received a half dose (LD) for their prime. For the other J o u r n a l P r e -p r o o f participants of COV0002 the median interval between the prime and boost was 69 days. For the COV003 study, all participants received or are receiving two shots of the vaccine at a dose of 3.5-6.5 × 10 10 viral particles with administration up to 12 weeks apart (median interval of 36 days). The total efficacy when both the low dose followed by standard dose (LD/SD) and 2 standard doses (SD/SD) participants were combined was 70.4% (95.8% CI 54.8-80.6, whereas for SD/SD participants it was 62.1% (95% CI 41.0-75.7) and for LD/SD it was 90.0% (95% CI 67.4-97.0). There was a non-significant increase in efficacy when the interval between prime and boost was >6 weeks (65.4%) as opposed to a gap of <6 weeks (53.4%). A subset of participants was also screened for asymptomatic infection, and the LD/SD group had an efficacy of 58.9% (95% CI 1.0%-82.9%) versus 3.8% (95% CI -72.4% to 46.3%) for the SD/SD group.

Though the vaccine has similar immunogenicity and better reactogenicity profile in older adults (>55 years old) [243] , not enough cases were accrued to assess efficacy in this age bracket due after vaccination. NAb titers were 61% and 77% for rAd26-S and rAd5-S vaccinated groups, respectively (these numbers were obtained by the authors after pooling data from the lyophilized and the frozen groups, since n was small in each group and there was no significant difference in response between the two vaccine formulations). For the Phase II participants, the seroconversion rate was also 100% when measured by RBD binding antibodies. 100% of participants in the prime-dose regimen of the Phase II study developed nAb titers. There was no significant correlation between baseline nAb titer against the viral vector and the titer of RBDspecific IgGs elicited after vaccination. Also, vaccination with one recombinant vector did not boost titers for the other vector, demonstrating lack of cross-reactivity between the two distinct viral vectors. Antigen-specific CD4+ and CD8+ T cell responses were observed following PBMC stimulation (samples collected after primary immunization) as measured by flow cytometry and IFN- secretion in 100% of the volunteers, particularly by d28. Significant cell proliferation was also observed in response to S protein stimulation. While IFN- is a marker of Th1-biased cellular responses, Phase III clinical trials will be further supplemented with more focus on Th1 and Th2 polarization. Data from the small number of participants showed that frozen and lyophilized vaccine formulations were equally immunogenic. Sputnik V received a highly controversial approval from Russian regulatory authorities, limiting rollout of the vaccine in the general population without a larger Phase III study. According to Gamaleya, the vaccine was used in high risk "red zones" of Russian hospitals. To date, the release says 10,000 people have received the vaccine under this authorization and the efficacy rate was more than 90%, but no data has been published to support these claims [245] . times the GMT of the convalescent serum panel, respectively, among participants 18 to 55 years of age. In those 65 to 85 years of age, the anti-S1 IgG response and the 50% neutralizing GMTs were 9.5 and 2.2 times the GMT of the convalescent serum panel, respectively. The vaccine was less immunogenic in the older cohort both for binding IgG and neutralization titers across dose levels on day 28. As reported in a preprint, antigen-specific CD8+ and Th1-type (IFN-+) CD4+ T cell responses were observed in most participants who received two 30g doses of BNT162b2. A poly-epitopic CD4+ T cell response directed against both N-and Cterminal portions of the S protein was observed. The magnitude of S-specific CD4+ and CD8+ T cell responses correlated with S1-binding IgG, indicating a convergent development of the humoral and cellular adaptive immunity upon vaccination. In 3 participants, the CD8+ T cell response was also evaluated using pMHC-tetramers. The CD8+ T cells showed an earlydifferentiated effector-memory phenotype (CCR7 lo CD45RA lo ) and markers associated with cognate activation (CD38, HLA-DR and PD-1), with single specificities reaching 0.01-3% of circulating CD8+ T cells [254] .

Most of the AEs reported were mild to moderate in severity, showed dose-dependence and increased in incidence after the second injection. Though a small fraction of adults in the younger cohort developed severe systemic events, none of the older participants reported any severe symptom. None of the participants reported grade four local or systemic AEs [255] .

Phase II/III: case in the vaccine group as opposed to 9 cases in the placebo group. Though adolescents aged 12-15 years were included in the trial, efficacy and immunogenicity data from this cohort has not been reported in this study [256] . BNT162b2 was authorized for use under an EUA by the FDA on December 11, 2020 for active immunization to prevent COVID-19 in individuals 16

years of age and older [257] .

Platform: recombinant protein nanoparticle plus Matrix-M1 adjuvant

Vaccine target: Trimeric SARS-CoV-2 S protein designed to be stabilized in the pre-fusion conformation Route of administration: intramuscular NVAX-CoV2373 is a protein subunit vaccine based on a full-length S, including the transmembrane and the cytoplasmic tail. To stabilize the prefusion conformation, the polybasic cleavage site was removed, and proline substitutions were introduced, as discussed above. The S protein was expressed and purified from insect cells in the presence of detergent which led to some higher order nanoparticle formation, as determined using electron microscopy. Imaging revealed that the antigen exists as mixture of free trimers as well as higher order multi-trimer rosettes [258] . Importantly, use of Matrix-M adjuvant with NVX-CoV2373 lead to significantly higher anti-S IgG titers across all dose groups when compared to mice immunized with 10μg NVX-CoV2373 alone [259] .

Challenge studies were performed in mice that were immunized with a similar regimen as For the Phase I trial study participants were divided into multiple groups receiving 5 or 25ug NVX-CoV2373 with or without Matrix-M in a prime only or prime-boost schedule.

Anti-spike IgG were detected in all adjuvanted (5 or 25ug) groups after the first vaccination with 10-fold higher levels in those receiving adjuvant as compared to the unadjuvanted group. A single vaccination with adjuvant (both 5 or 25ug dose groups) resulted in IgG titers that were similar to those from COVID-19 patients who had subclinical infections. No significant differences were observed in the antibody responses between a prime-boost regimen with 5μg or 25μg NVX-CoV2373 plus Matrix-M. T-cell responses measured by ICS following a 6h stimulation with rSARS-CoV-2 in 16 randomly selected participants from select groups showed that adjuvanted regimens induced antigen-specific polyfunctional CD4+ T-cell responses at d28

which were skewed to a Th1 phenotype (IFN-, TNF-, IL-2).

Reactogenicity was largely absent or mild and self-limited. Mild unsolicited AEs were reported that were similarly distributed across the groups receiving adjuvanted and unadjuvanted vaccine. No severe AEs were reported [228] .

Phase IIa/b While a dose-dependent increase in the immunogenicity profile was consistently observed, the higher doses were also associated with an increased incidence of AEs and thus often discontinued in Phase III trials. The early phases of testing demonstrate the need for evaluation of multiple dosages to balance the trade-off between dose sparing and immunogenicity. Another feature that is being observed consistently across all the different vaccine platforms from Phase I/II trials is that the optimal window for a booster immunization is generally between 21 to 28 days, consistent with previous knowledge in human vaccinology. A second immunization at shorter intervals (<21 days) often did not result in a significant increase in antigen-specific antibody titers [47, 154, 237] . All of the vaccine candidates induced high-levels of seroconversion in healthy adults, independent of assay variability, and in the vast majority cases, the early neutralization titers correlated well with the binding antibody titers.

J o u r n a l P r e -p r o o f

The early data from clinical trials of SARS-CoV-2 vaccine candidates are very encouraging.

While these reports give hope that multiple vaccines will soon be available to prevent infections in many people, one significant issue with the vast majority of ongoing Phase III studies is absence of diversity in study cohorts and little or no representation of populations with underlying conditions that confer risk for severe COVID-19. While the vaccines have thus far been reasonably safe and well tolerated, safety profiles in populations that are at highest riskthose other than healthy adultsremain unknown.

Another caveat in the vaccine development pipeline is that commonly used measures of immunogenicity are often extrapolated to make predictions about efficacy. Though these readouts give some important information about the vaccine candidate, their relevance for predicting vaccine efficacy in humans is speculative. Furthermore, determinants of protection are likely to vary between groups of people based on age and other health considerations [261] .

In addition to safety and efficacy, the durability of vaccine responses is an extremely important consideration for vaccine selection, yet it is one that will not be a factor in the selection of the first SARS-CoV-2 vaccines for use in humans.

The most important next step is to define these determinants of immunity and the viral targets of protective immune responses against SARS-CoV-2. Based on current published literature, a robust neutralizing antibody response against the S protein is ideal for providing sterilizing immunity. Thus, a majority of the initial analyses of immune correlates of protection have been focused on the S protein. However, our understanding of host factors and viral targets that provide immunity against SARS-CoV-2 infection or disease remains incomplete. These are also likely to vary among people based on factors such as age, sex, and comorbidities.

While the first wave of vaccines in development is mostly focused on eliciting neutralizing antibodies, a longer-term goal for SARS-CoV-2 vaccine design is to elicit broad immunity against distinct virus variants and strains. Early reports have found little antigenic variation among SARS-CoV-2 isolates and sequencing shows a rate of genetic variation in SARS-CoV-2 that is 2 to 6-fold lower than that observed in influenza viruses [262] . This is somewhat encouraging, though we expect that the rate of antigenic drift in SARS-CoV-2 will increase over J o u r n a l P r e -p r o o f time, under the selective pressure of neutralizing antibodies in the population. Therefore, development of broad vaccines against SARS viruses or more broadly against coronaviruses will be important. This will almost certainly require immune responses that can recruit specific cellular effector functions through broadly reactive, potentially non-neutralizing antibody responses and that engage T cell responses against conserved domains of coronavirus proteins [263] [264] [265] [266] [267] . A consideration here is that, unlike viruses that bud directly from the cell membranes Finally, SARS-CoV-2 vaccination can stem the pandemic only if enough of the population is vaccinated to achieve herd immunity. The threshold for herd immunity through vaccination is a function of R 0 of the pathogen. . For SARS-CoV-2, this is estimated to be 50%-67% for a population with no preexisting immunity and equal susceptibility [278] . The accuracy of this prediction is contingent not only on the ability to properly estimate the reproductive number, but also on the ability of the vaccine to prevent continued transmission. Thus, even with an extremely effective vaccine, uptake will need to be high, and likely greater than 67%, in order to achieve reasonable population immunity. On this topic, data from the Vaccine Confidence Project (VCP), which tracks the beliefs and attitudes of people around the world about vaccine safety and effectiveness, shows that we need to dramatically improve education about how vaccines work and how they have been used to control infectious diseases over the past ~100

years. Recent research from the VCP has shown that 83% of individuals in the United States are worried about the safety of vaccines that are currently being expedited for use [279] . This is not altogether surprising given the maelstrom of political rhetoric and misinformation that exists J o u r n a l P r e -p r o o f around SARS-CoV-2 in the media, yet it suggests that a challenge is on the horizon -to promote broad and expedient compliance with uptake of the new SARS-CoV-2 vaccines.

Nonetheless, the combined forces that have come together to stop the SARS-CoV-2 pandemic are virtually certain to result in multiple vaccines which, in the coming months, will be deployed to save lives.

Competing interest statement: the authors have no competing interests to declare.

Acknowledgments:

We thank JoEllen Barnett for helpful comments and suggestions. Support was received from [1] R. Ahmed, D.R. Burton, Viral vaccines: past successes and future challenges, Curr Opin Virol, 3 (2013) 307-308.

[2] E. Padron-Regalado, Vaccines for SARS-CoV-2: Lessons from Other Coronavirus Strains, Infect Dis Ther, (2020) 1-20.

[3] D. van Riel, E. de Wit, Next-generation vaccine platforms for COVID-19, Nat Mater, 19 (2020) 810-812.

[4] F. Krammer, SARS-CoV-2 vaccines in development, Nature, 586 (2020) 516-527.

Development and Licensure of Vaccines to Prevent

PFIZER AND BIONTECH CONCLUDE PHASE 3 STUDY OF COVID-19 VACCINE CANDIDATE

Moderna's COVID-19 Vaccine Candidate Meets its Primary Efficacy Endpoint in the First Interim Analysis of the Phase 3 COVE Study

DRAFT landscape of COVID-19 candidate vaccines

Vaccines and Related Biological Products Advisory Committee Meeting December

Vaccines and Related Biological Products Advisory Committee Meeting December

Insights into SARS-CoV-2 genome, structure, evolution, pathogenesis and therapies: Structural genomics approach

A new coronavirus associated with human respiratory disease in China

Virology, transmission, and pathogenesis of SARS-CoV-2

The Lancet Respiratory, COVID-19 transmission-up in the air

Airborne Transmission of SARS-CoV-2: Theoretical Considerations and Available Evidence

Reducing transmission of SARS-CoV-2

Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1

Probable Evidence of Fecal Aerosol Transmission of SARS-CoV-2 in a High-Rise Building

Scientific brief: SARS-CoV-2 and potential airborne transmission

The presence of SARS-CoV-2 RNA in the feces of COVID-19 patients

State -nCo, First Case of 2019 Novel Coronavirus in the United States

Infectious SARS-CoV-2 in Feces of Patient with Severe COVID-19

The Incubation Period of Coronavirus Disease 2019 (COVID-19) From Publicly Reported Confirmed Cases: Estimation and Application

Early Transmission Dynamics in Wuhan, China, of Novel Coronavirus-Infected Pneumonia

Occurrence and transmission potential of asymptomatic and presymptomatic SARS-CoV-2 infections: A living systematic review and meta-analysis

Projecting the transmission dynamics of SARS-CoV-2 through the postpandemic period

Preliminary prediction of the basic reproduction number of the Wuhan novel coronavirus 2019-nCoV

Towards an accurate and systematic characterisation of persistently asymptomatic infection with SARS-CoV-2

Defining the role of asymptomatic and pre-symptomatic SARS-CoV-2 transmission -a living systematic review, medRxiv

Genome Composition and Divergence of the Novel Coronavirus (2019-nCoV) Originating in China

A SARS-CoV-2 protein interaction map reveals targets for drug repurposing

Accessory proteins of SARS-CoV and other coronaviruses

SARS coronavirus accessory proteins

Dynamically evolving novel overlapping gene as a factor in the SARS-CoV-2 pandemic

Severe acute respiratory syndrome coronavirus 3a protein is a viral structural protein

Severe acute respiratory syndrome coronavirus 7a accessory protein is a viral structural protein

Mapping Neutralizing and Immunodominant Sites on the SARS-CoV-2 Spike Receptor-Binding Domain by Structure-Guided High-Resolution Serology

Cross-neutralization of SARS-CoV-2 by a human monoclonal SARS-CoV antibody

Structural insights into coronavirus entry

Potent neutralizing antibodies against multiple epitopes on SARS-CoV-2 spike

Neutralizing Antibodies Correlate with Protection from SARS-CoV-2 in Humans during a Fishery Vessel Outbreak with a High Attack Rate

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Safety and immunogenicity of the Ad26.COV2.S COVID-19 vaccine candidate: interim results of a phase 1/2a, double-blind, randomized

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A noncompeting pair of human neutralizing antibodies block COVID-19 virus binding to its receptor ACE2

Structure, Function, and Antigenicity of the SARS-CoV-2 Spike Glycoprotein

Cryo-EM structure of the 2019-nCoV spike in the prefusion conformation

Tectonic conformational changes of a coronavirus spike glycoprotein promote membrane fusion

Bottcher-Friebertshauser, TMPRSS2 and furin are both essential for proteolytic activation of SARS-CoV-2 in human airway cells

Ready, set, fuse! The coronavirus spike protein and acquisition of fusion competence

Host cell proteases: Critical determinants of coronavirus tropism and pathogenesis

Distinct conformational states of SARS-CoV-2 spike protein

Unexpected Receptor Functional Mimicry Elucidates Activation of Coronavirus Fusion

Convergent antibody responses to SARS-CoV-2 in convalescent individuals

Serology assays to manage COVID-19

Immunogenicity and structures of a rationally designed prefusion MERS-CoV spike antigen

Stabilized coronavirus spikes are resistant to conformational changes induced by receptor recognition or proteolysis

Cryo-electron microscopy structure of a coronavirus spike glycoprotein trimer

Structural basis for human coronavirus attachment to sialic acid receptors

Crucial steps in the structure determination of a coronavirus spike glycoprotein using cryoelectron microscopy

Structure of bacteriophage T4 fibritin: a segmented coiled coil and the role of the C-terminal domain

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IgG Fc Glycosylation in Human Immunity

An inflammatory cytokine signature predicts COVID-19 severity and survival

IgG antibodies to dengue enhanced for FcgammaRIIIA binding determine disease severity

Coronavirus envelope protein: current knowledge

Role of Severe Acute Respiratory Syndrome Coronavirus Viroporins E, 3a, and 8a in Replication and Pathogenesis

Coronavirus E protein forms ion channels with functionally and structurally-involved membrane lipids

Severe acute respiratory syndrome coronavirus envelope protein ion channel activity promotes virus fitness and pathogenesis

Nucleocapsid-independent assembly of coronavirus-like particles by co-expression of viral envelope protein genes

Generation of synthetic severe acute respiratory syndrome coronavirus pseudoparticles: implications for assembly and vaccine production

Dissection and identification of regions required to form pseudoparticles by the interaction between the nucleocapsid (N) and membrane (M) proteins of SARS coronavirus

Virus-like particles of SARS-like coronavirus formed by membrane proteins from different origins demonstrate stimulating activity in human dendritic cells

Assembly of severe acute respiratory syndrome coronavirus RNA packaging signal into virus-like particles is nucleocapsid dependent

Assembly of human severe acute respiratory syndrome coronavirus-like particles

Biochemical evidence for the presence of mixed membrane topologies of the severe acute respiratory syndrome coronavirus envelope protein expressed in mammalian cells

Characterization of the coronavirus mouse hepatitis virus strain A59 small membrane protein E

Subcellular location and topology of severe acute respiratory syndrome coronavirus envelope protein

The missing link in coronavirus assembly. Retention of the avian coronavirus infectious bronchitis virus envelope protein in the pre-Golgi compartments and physical interaction between the envelope and membrane proteins

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The SARS coronavirus nucleocapsid protein--forms and functions

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Severe acute respiratory syndrome coronavirus ORF3a protein activates the NLRP3 inflammasome by promoting TRAF3-dependent ubiquitination of ASC

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Structure and intracellular targeting of the SARS-coronavirus Orf7a accessory protein

Severe acute respiratory syndrome coronavirus gene 7 products contribute to virus-induced apoptosis

SARS coronavirus accessory ORFs encode luxury functions

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Differential stepwise evolution of SARS coronavirus functional proteins in different host species

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A potential molecular mechanism for hypersensitivity caused by formalininactivated vaccines

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Principles underlying rational design of live attenuated influenza vaccines

Adverse events following immunization in patients with primary immunodeficiencies

Cutaneous and Visceral Chronic Granulomatous Disease Triggered by a Rubella Virus Vaccine Strain in Children With Primary Immunodeficiencies

Balancing the Efficacy and Safety of Vaccines in the Elderly

Viral vectors for vaccine applications

Developments in Viral Vector-Based Vaccines, Vaccines (Basel)

Factors Which Contribute to the Immunogenicity of Non-replicating Adenoviral Vectored Vaccines, Front Immunol

New Vaccine Technologies to Combat Outbreak Situations

Noninvasive vaccination against infectious diseases

Intranasal and oral vaccination with protein-based antigens: advantages, challenges and formulation strategies

Step Study Protocol, Efficacy assessment of a cellmediated immunity HIV-1 vaccine (the Step Study): a double-blind, randomised, placebocontrolled, test-of-concept trial

A replication defective recombinant Ad5 vaccine expressing Ebola virus GP is safe and immunogenic in healthy adults

Adenoviral vector immunity: its implications and circumvention strategies

Pre-existing immunity against Ad vectors: humoral, cellular, and innate response, what's important?

A single dose of ChAdOx1 MERS provides protective immunity in rhesus macaques

A Single-Dose Intranasal ChAd Vaccine Protects Upper and Lower Respiratory Tracts against SARS-CoV-2

Regulatory aspects of quality and safety for live recombinant viral vaccines against infectious diseases in Japan

Department of Defense Smallpox Vaccination Clinical Evaluation, Incidence and follow-up of inflammatory cardiac complications after smallpox vaccination

Chimpanzee Adenovirus Vector Ebola Vaccine

Ebola vaccines in clinical trial: The promising candidates

A Monovalent Chimpanzee Adenovirus Ebola Vaccine Boosted with MVA

Immunogenicity, safety, and tolerability of a recombinant measlesvirus-based chikungunya vaccine: a randomised, double-blind, placebo-controlled, activecomparator, first-in-man trial

Single-shot live-attenuated chikungunya vaccine in healthy adults: a phase 1, randomised controlled trial

Immunogenicity and safety of a recombinant adenovirus type-5-vectored COVID-19 vaccine in healthy adults aged 18 years or older: a randomised, double-blind

Development of a Multi-Antigenic SARS-CoV-2 Vaccine Using a Synthetic Poxvirus Platform

Control of recombinant monoclonal antibody effector functions by Fc N-glycan remodeling in vitro

Modified Vaccinia Ankara Based SARS-CoV-2 Vaccine Expressing Full-Length Spike Induces Strong Neutralizing Antibody Response, bioRxiv

Modified Vaccinia Ankara Based SARS-CoV-2 Vaccine Expressing Full-Length Spike Induces Strong Neutralizing Antibody Response, bioRxiv

Merck, one of Big Pharma's biggest players, reveals its COVID-19 vaccine and therapy plans

A single dose of recombinant VSV-ΔG-spike vaccine provides protection against SARS-CoV-2 challenge, bioRxiv

A Highly Immunogenic Measles Virus-based Th1-biased COVID-19

Disease Virus (NDV) Expressing a Membrane-Anchored Spike as a Cost-Effective Inactivated SARS-CoV-2 Vaccine, Vaccines (Basel)

Phase 1 clinical trials of the safety and immunogenicity of adjuvanted plasmid DNA vaccines encoding influenza A virus H5 hemagglutinin

DNA priming and influenza vaccine immunogenicity: two phase 1 open label randomised clinical trials, The Lancet. Infectious diseases

DNA vaccination protects against an influenza challenge in a double-blind randomised placebo-controlled phase 1b clinical trial

Intradermal SynCon(R) Ebola GP DNA Vaccine Is Temperature Stable and Safely Demonstrates Cellular and Humoral Immunogenicity Advantages in Healthy Volunteers

Preliminary aggregate safety and immunogenicity results from three trials of a purified inactivated Zika virus vaccine candidate: phase 1, randomised, double-blind, placebo-controlled clinical trials

Safety and immunogenicity of an anti-Middle East respiratory syndrome coronavirus DNA vaccine: a phase 1, open-label, single-arm, doseescalation trial

Harnessing Recent Advances in Synthetic DNA and Electroporation Technologies for Rapid Vaccine Development Against COVID-19 and Other Emerging Infectious Diseases

Immunogenicity of a DNA vaccine candidate for COVID-19

Intradermal-delivered DNA vaccine provides anamnestic protection in a rhesus macaque SARS-CoV-2 challenge model, bioRxiv

COVID-19 and mRNA Vaccines-First Large Test for a New Approach

Self-amplifying RNA SARS-CoV-2 lipid nanoparticle vaccine candidate induces high neutralizing antibody titers in mice

m, An mRNA Vaccine against SARS-CoV-2 -Preliminary Report

mRNA vaccines -a new era in vaccinology

Advances in mRNA Vaccines for Infectious Diseases

mRNA as a Transformative Technology for Vaccine Development to Control Infectious Diseases

Temperature concerns could slow the rollout of new coronavirus vaccines

The promise of mRNA vaccines: a biotech and industrial perspective

Zika virus protection by a single low-dose nucleoside-modified mRNA vaccination

Modified mRNA Vaccines Protect against Zika Virus Infection

Non-conventional expression systems for the production of vaccine proteins and immunotherapeutic molecules

Recombinant vaccines and the development of new vaccine strategies

Elicitation of Potent Neutralizing Antibody Responses by Designed Protein Nanoparticle Vaccines for SARS

A single immunization with spike-functionalized ferritin vaccines elicits neutralizing antibody responses against SARS-CoV-2 in mice, bioRxiv

Flucelvax (Optaflu) for seasonal influenza

FluBlok, a recombinant hemagglutinin influenza vaccine, Influenza Other Respir Viruses

Advisory Committee on Immunization, A comprehensive immunization strategy to eliminate transmission of hepatitis B virus infection in the United States: recommendations of the Advisory Committee on Immunization Practices (ACIP) part 1: immunization of infants, children, and adolescents

Phase 1-2 Trial of a SARS-CoV-2 Recombinant Spike Protein Nanoparticle Vaccine

Engineering virus-like particles as vaccine platforms

Virus-Like Particles as an Instrument of Vaccine Production

Virus-like particles: the future of microbial factories and cell-free systems as platforms for vaccine development

Virus-like particles in vaccine development

Phase 1 trial of a Candidate Recombinant Virus-Like Particle Vaccine for Covid-19 Disease Produced in Plants, medRxiv

Tracking Changes in SARS-CoV-2 Spike: Evidence that D614G Increases Infectivity of the COVID-19 Virus

Altered reactivity to measles virus. Atypical measles in children previously immunized with inactivated measles virus vaccines

Respiratory syncytial virus disease in infants despite prior administration of antigenic inactivated vaccine

Safety, tolerability, and immunogenicity of an inactivated SARS-CoV-2 vaccine in healthy adults aged 18-59 years: a randomised, double-blind, placebo-controlled, phase 1/2 clinical trial

ChAdOx1 nCoV-19 vaccine prevents SARS-CoV-2 pneumonia in rhesus macaques

T cell and antibody responses induced by a single dose of ChAdOx1 nCoV-19 (AZD1222) vaccine in a phase 1/2 clinical trial

Safety and immunogenicity of ChAdOx1 nCoV-19 vaccine administered in a prime-boost regimen in young and old adults (COV002): a single-blind

AstraZeneca's COVID-19 vaccine authorised for emergency supply in the UK

Russia's claim of a successful COVID-19 vaccine doesn't pass the 'smell test,' critics say

SARS-CoV-2 mRNA vaccine design enabled by prototype pathogen preparedness

Evaluation of the mRNA-1273 Vaccine against SARS-CoV-2 in Nonhuman Primates

m, Durability of Responses after SARS-CoV-2 mRNA-1273 Vaccination

Efficacy and Safety of the mRNA-1273 SARS-CoV-2 Vaccine

Advisory Committee on Immunization Practices Recommends Vaccination with Moderna's COVID-19 Vaccine for Persons 18 Years and Older

Phase I/II study of COVID-19 RNA vaccine BNT162b1 in adults

A prefusion SARS-CoV-2 spike RNA vaccine is highly immunogenic

BNT162b2 induces SARS-CoV-2-neutralising antibodies and T cells in humans

Safety and Immunogenicity of Two RNA-Based Covid-19 Vaccine Candidates

Safety and Efficacy of the BNT162b2 mRNA Covid-19 Vaccine

CDC COMMITTEE OF INDEPENDENT HEALTH EXPERTS RECOMMENDS VACCINATION WITH PFIZER AND BIONTECH COVID-19 VACCINE FOR PERSONS AGES 16 YEARS AND OLDER

Structural analysis of full-length SARS-CoV-2 spike protein from an advanced vaccine candidate

SARS-CoV-2 spike glycoprotein vaccine candidate NVX-CoV2373 elicits immunogenicity in baboons and

NVX-CoV2373 vaccine protects cynomolgus macaque upper and lower airways against SARS-CoV-2 challenge

On the evolutionary epidemiology of SARS-CoV-2

T cell responses in patients with COVID-19

Proinflammatory IgG Fc structures in patients with severe COVID-19

Broadly neutralizing anti-influenza antibodies require Fc receptor engagement for in vivo protection

Broadly neutralizing hemagglutinin stalkspecific antibodies require FcgammaR interactions for protection against influenza virus in vivo

Both Neutralizing and Non-Neutralizing Human H7N9 Influenza Vaccine-Induced Monoclonal Antibodies Confer Protection

The Role of Fc Gamma Receptors in Broad Protection against Influenza Viruses, Vaccines (Basel

Hemagglutinin stalk antibodies elicited by the 2009 pandemic influenza virus as a mechanism for the extinction of seasonal H1N1 viruses

Antibody recognition of a highly conserved influenza virus epitope

Structural and functional bases for broad-spectrum neutralization of avian and human influenza A viruses

Protective efficacy of influenza group 2 hemagglutinin stem-fragment immunogen vaccines

Increasing the breadth and potency of response to the seasonal influenza virus vaccine by immune complex immunization

Optimal activation of Fc-mediated effector functions by influenza virus hemagglutinin antibodies requires two points of contact

A. Lanzavecchia, A neutralizing antibody selected from plasma cells that binds to group 1 and group 2 influenza A hemagglutinins

Influenza hemagglutinin stem-fragment immunogen elicits broadly neutralizing antibodies and confers heterologous protection

Influenza viruses expressing chimeric hemagglutinins: globular head and stalk domains derived from different subtypes

Herd Immunity and Implications for SARS-CoV-2 Control