key: cord-0018266-1esowken
authors: Kim, Yun‐Hee; Hong, Kee‐Jong; Kim, Hun; Nam, Jae‐Hwan
title: Influenza vaccines: Past, present, and future
date: 2021-05-04
journal: Rev Med Virol
DOI: 10.1002/rmv.2243
sha: 53e2984ceb5b49851602f9509acef6318e8f11d5
doc_id: 18266
cord_uid: 1esowken

Globally, infection by seasonal influenza viruses causes 3–5 million cases of severe illness and 290,000–650,000 respiratory deaths each year. Various influenza vaccines, including inactivated split‐ and subunit‐type, recombinant and live attenuated vaccines, have been developed since the 1930s when it was discovered that influenza viruses could be cultivated in embryonated eggs. However, the protection rate offered by these vaccines is rather low, especially in very young children and the elderly. In this review, we describe the history of influenza vaccine development, the immune responses induced by the vaccines and the adjuvants applied. Further, we suggest future directions for improving the effectiveness of influenza vaccines in all age groups. This includes the development of an influenza vaccine that induces a balanced T helper cell type 1 and type 2 immune responses based on the understanding of the immune system, and the development of a broad‐spectrum influenza vaccine that can increase effectiveness despite antigen shifts and drifts, which are characteristics of the influenza virus. A brighter future can be envisaged if the development of an adjuvant that is safe and effective is realized.

marketed preventive vaccines differ in type (whole, split, recombinant and subunit inactivated, and live-attenuated types) and the substrate used for production (embryonated eggs or cells). 15 A study on vaccine effectiveness by the USA Centers for Disease Control and Prevention showed that their protection rate remains low (40%-60%), despite being antigenically matched with circulating strains. 16 Of concern, vaccines do not effectively elicit immune responses among very young children (6-35 months) and elderly people (>65 years), 17, 18 the two age groups more vulnerable to influenza virus infection and with higher mortality rates. Therefore, there is an urgent need for effective influenza vaccines for all ages.

Methods for culturing influenza viruses were developed in the 1930s.

In 1933, the human influenza virus was transmitted to ferrets by intranasal instillation of a specimen obtained from throat washings collected from a patient. 19 In 1935, Wilson Smith suggested a method for cultivating influenza virus in the chorioallantoic membrane of embryonated eggs. 20 This method yielded substantially higher virus concentrations than previous methods based on virus extraction from the lungs of infected animals. Embryonated eggs are still used today to produce influenza vaccines. In addition, cultivation methods using cells and medium have been developed since the mid-1930s. 21, 22 With the ability to isolate and culture influenza viruses, research on influenza vaccine development took off. From the mid-1930s to the early 1940s, vaccine effects of activated and formalin-inactivated influenza viruses obtained from allantoic fluid of embryonated eggs or extracts of infected animal organs were studied in animals and humans by monitoring antibody production. [23] [24] [25] It was found that antigenic matching between the vaccine and circulating strains is important to guarantee vaccine efficacy 26 and that concentrated vaccine is more effective than the unconcentrated vaccine, whether or not the virus is inactivated. 27, 28 In 1942, immune responses induced by an inactivated whole bivalent influenza vaccine consisting of the PR8 strain of influenza type A and the Lee strain of influenza type B and produced in embryonated eggs were evaluated in a clinical study. 29 In 1943, a larger clinical study by the Commission on Influenza of the U.S. Armed Forces showed that inactivated whole trivalent vaccine including the A-subtype PR8 and Weiss strains and the B-type Lee strain protected against influenza. 30 Vaccine doses were established through clinical trials, and in 1945, the first inactivated influenza vaccine (IIV) was licensed in the United States. 31 Since the development of the first inactivated whole virus vaccine in embryonated chicken eggs in the 1940s, production methods for IIVs were continuously improved, and in the 1950s, the current IIV manufacturing process using embryonated eggs was developed. 32 IIVs included whole-virus, split-virus disrupted by a detergent and further purified subunit vaccines composed of surface antigen, HA and NA. 33 The whole-virus vaccine, the first developed IIV, induced good immune responses even in unprimed individuals. 34 However, there were concerns about pyrogenicity and adverse side effects. 35 To overcome these problems, in the 1960s, split virus vaccines were developed by treating the virus with ether or detergent, 36 which made them safe for children. 37 In the 1970s, purified subunit influenza vaccines mainly based on HA and NA were developed, which further improved safety and reduced reactogenicity. 34, 35, [38] [39] [40] [41] Live-attenuated influenza vaccines (LAIVs), prepared by successive passages of influenza virus in ferrets and mice or embryonated eggs, have also been studied since the 1930s. 42 These host-range variant vaccines protect against influenza without causing flu symptoms in humans. However, they have some drawbacks, including low virus titres and difficulties in maintaining constant attenuation and antigenicity levels. 43 In the 1960s, a new method was adopted to attenuate influenza virus through consecutive passages in embryonated eggs at low temperatures, yielding cold-adapted, temperaturesensitive variants. 44 Since cold-adapted, temperature-sensitive influenza virus replicates best at lower temperatures, viral replication was enacted in the nasal cavity and not in the respiratory tract.

These viruses were safer than those attenuated by previous methods and induced an immune response. Accordingly, they were further developed as donor viruses for LAIVs. 45 As RNA viruses, influenza viruses lack proofreading activity and therefore are genetically unstable; thus, antigenic mutations occur at a high frequency. 46 To increase the protection rate of influenza vaccines, it is important to accurately predict the influenza viruses that will circulate and to manufacture vaccines with those strains. To this end, the World Health Organization's (WHO) Global Influenza Surveillance and Response System has conducted global influenza surveillance since 1952, and based on the monitoring results, the WHO annually announces recommended influenza virus vaccine compositions for the northern (February) and southern (September) hemispheres. 47, 48 Initially, the WHO recommended three influenza virus strains, including A/H1N1, A/H3N2, and either the B/Victoria or the B/ Yamagata lineage for trivalent vaccines; however, since co-circulation of the two B lineages was observed at a high frequency, B/Victoria as well as B/Yamagata are being recommended for quadrivalent influenza vaccines. 49 However, since IIVs and LAIVs are manufactured using the recommended candidate viruses, they will not be effective because of antigenic mismatch if the prediction is not accurate. Thus, broadspectrum or universal influenza vaccines are being actively researched. These vaccine types target a conserved region of the influenza virus, such as the stalk region of HA, M2e, M1 or nucleoprotein instead of the globular head of HA, which is immunodominant, but variable and strain-specific ( Figure 1) . 15, 50 These vaccines are prepared by using a viral vector, DNA vector, virus-like particle (VLP), nanoparticle or a peptide that directly stimulates T cells. 51 

Although the conventional method using embryonated eggs is still predominantly used worldwide, it has significant drawbacks.

Egg-based vaccines may cause an allergic reaction to albumin, and when the demand for embryonated eggs suddenly increases, for example, during a pandemic, the supply may be insufficient, hampering timely vaccine production. 54 63, 64 Thus, influenza vaccine development is gradually moving away from the conventional egg-based platform to the cell culture ( Figure 2 ), though efforts to increase yield and lower production costs of the latter are needed. Among the various subsets of CD4+ effector T cells, Th1, Th2 and Th17 cells play major roles in defense against pathogens.

Th1 cells, which induce a cell-mediated immune response, secrete IFN-γ and tumour necrosis factor-alpha (TNF-α), which activate macrophages and neutrophils that eliminate intracellular pathogens F I G U R E 1 Targets of broad-spectrum or universal influenza vaccine. Broad-spectrum or universal influenza vaccines target antigens that can elicit broadly cross-reactive immune responses. Antibodies induced by HA stalk and ectodomain of the M2 ion channel (M2e), which are highly conserved regions, can mediate antibody-dependent, cell-mediated cytotoxicity (ADCC). Antibodies against HA stalk is neutralizing; while antibodies against M2e is not. M1 and NP proteins possess conserved regions and are internal proteins. Therefore, they mainly induce cytotoxic T cell responses KIM ET AL. While alum has since long been widely used as an adjuvant, its mechanism remains unknown. 92 Originally, alum was thought to enhance immune responses by slowly and continuously exposing the antigen to APCs through the so-called antigen depot effect. 93 However, recent studies showed that alum also recruits various types of innate immune cells, including neutrophils and monocytes, to the injection site, thereby activating innate immune responses. [94] [95] [96] Studies on the cellular and molecular mechanisms have reported that alum triggers the activation of the NLRP3 inflammasome through phagosomal destabilization via alum phagocytosis by APCs, resulting in the secretion of the pro-inflammatory cytokine IL-1β. [97] [98] [99] [100] In addition, alum causes necrosis at the injection site, resulting in the release of damage-associated molecular patterns, such as uric acid and DNA, which activate the NLRP3 inflammasome. 95, 101 Alum can enhance the immune response through prostaglandin E 2 production, which is involved in inducing a Th2 immune response, via ITAM-Syk-PI3Kδ signalling. 102 These results suggest that alum induces a Th2 response rather than a Th1 response.

MF59 is an oil-in-water adjuvant consisting of squalene and two surfactants, polysorbate 80 (Tween 80) and sorbitan trioleate (Span 85). Since its first application in influenza vaccines in Europe in 1997, substantial research has been done to clarify its immune-boosting mechanism. MF59 was shown to have no depot effect, 103,104 which was supported by the finding that it showed an adjuvant effect even when injected 24 h before to 1 h after antigen injection. 105 Various emulsion-type adjuvants with different components and ratios are in clinical trials (Table 3) . [123] [124] [125] [126] Other particle types, such as virosome or VLP, may also have an adjuvant function. 127 Several immunostimulators of which the mechanisms of action are not yet known are also being studied. [128] [129] [130] By the way, cytokine, which stimulates the Th1 immune response and B lymphocyte differentiation in mice, showed no adjuvant effect in phase I clinical trials. 131 Thus, it is important to develop a non-clinical system that can accurately predict adjuvant effects. In a recent study, an IIV formulated with a single-stranded RNA adjuvant induced cross-protection against heterologous influenza virus infection and mucosal immune response. 133 The detailed mechanism and safety aspects remain to be studied.

The influenza vaccines currently on the market can be administered to very young children (≥6 months of age), although the recom- with a smaller amount of HA antigen (9 µg per strain) compared to an intramuscular route. 137, 138 However, FDA-licensed products using these approaches are also not yet applicable to young children or the elderly ( Table 1 ). The LAIV FluMist is approved for use in persons 2-49 years of age, and Fluzone intradermal, which is injected intradermally, is approved for use in persons 18-64 years of age.

The adverse events associated with the influenza vaccine vary from mild symptoms, such as erythema from the shot, headache, fever, nausea, and myalgia to unusual events, such as severe allergic reaction, Guillain-Barré syndrome and oculo-respiratory syndrome.

Most of the adverse events associated with influenza vaccines are mild and easy to recover. 139 RNA-based adjuvants. These are as easily degradable as TLR3 or TLR7/8 ligands 133 and are relatively safe, when compared to other adjuvants that remain in the body for long time.

Influenza vaccines are somewhat complex. They induce different immune responses depending on the vaccine type, such as IIV versus LAIV, and the age at vaccination and their mechanisms in inducing immune responses have not been completely clarified.

Most of the licensed vaccines induce mainly Th2-type immune responses, and in young children and the elderly, they induce weaker immune responses than in adults. Therefore, efforts are needed to develop influenza vaccines that induce stronger and more balanced Th1/Th2 immune responses, based on our understanding of the immune system, which differs according to age.

Th2 responses, will have to be modified accordingly to include confirmation of T-cell-mediated protection. Improved immune responses can also be achieved by using adjuvants, and thus, safer and more effective adjuvants should be developed. Finally, effective and reliable tools for predicting immune responses to vaccines and adjuvants would greatly help increase the protection rate against influenza virus infection, not only in adults but also in young children and the elderly.

The biology of influenza viruses

Isolation of a novel swine influenza virus from Oklahoma in 2011 which is distantly related to human influenza C viruses

Epidemiological and virological characterization of influenza B virus infections

Influenza vaccine: an effective preventive vaccine for developing countries

Influenza Type A Viruses

New world bats harbor diverse influenza A viruses

The evolution of seasonal influenza viruses

WHO Ask The Expert: Influenza Q&A

Estimates of US influenza-associated deaths made using four different methods. Influenza and other respiratory viruses

Global burden of respiratory infections due to seasonal influenza in young children: a systematic review and meta-analysis

Influenza Antiviral Medications: Summary for Clinicians

Validation of the safety of MDCK cells as a substrate for the production of a cell-derived influenza vaccine

Evolution and ecology of influenza A viruses

The global circulation of seasonal influenza A (H3N2) viruses

History and evolution of influenza control through vaccination: from the first monovalent vaccine to universal vaccines

CDC Seasonal Flu Vaccine Effectiveness Studies

Antibody response to influenza vaccination in the elderly: a quantitative review

Evaluation of the influenza vaccine effectiveness among children aged 6 to 72 months based on the test-negative case control study design. Zhonghua Yu Fang Yi Xue Za Zhi

A virus obtained from influenza patients

Cultivation of the virus of influenza

Cultivation of human influenza virus in an artificial medium

Studies with human influenza virus cultivated in artificial medium

Results of immunization by means of active virus of human influenza

Vaccination against epidemic influenza with active virus of human influenza. A two-year study

A study of epidemic influenza: with special reference to the 1936-7 epidemic. Special report series

An experiment in immunization against influenza with a formaldehyde-inactivated virus

Antibody response of human beings following vaccination with influenza viruses

A new method for concentrating influenza virus from allantoic fluid

Immunization against influenza with observations during an epidemic of influenza A one year after vaccination

A clinical, epidemiological and immunological evaluation of vaccination against epidemic influenza

Influenza Historic Timeline

Vaccines for Biodefense and Emerging and Neglected Diseases

Inactivated whole virus particle vaccine with potent immunogenicity and limited IL-6 induction is ideal for influenza

Summary of clinical trials of influenza virus vaccines in adults

A surface antigen influenza vaccine: 2. Pyrogenicity and antigenicity

Antibody response in humans to deoxycholate-treated influenza virus vaccine

Advances in the development of influenza virus vaccines

Preparation and properties of a novel influenza subunit vaccine

A surface antigen influenza vaccine. 1. Purification of haemagglutinin and neuraminidase proteins

A controlled double-blind comparison of reactogenicity, immunogenicity, and protective efficacy of whole-virus and split-product influenza vaccines in children

Influenza vaccine: split-product versus wholevirus types-how do they differ?

Investigation on volunteers infected with the influenza virus

Investigation into the attenuation of influenza viruses by serial passage

Adaptation and growth characteristics of influenza virus at 25 C

Safety, immunogenicity and infectivity of new live attenuated influenza vaccines

Influenza A virus polymerase: structural insights into replication and host adaptation mechanisms

65 years of influenza surveillance by a World Health Organization-coordinated global network. Influenza Other Respir Viruses

The WHO global influenza surveillance and response system (GISRS)-a future perspective. Influenza Other Respir Viruses

Public health impact of including two lineages of influenza B in a quadrivalent seasonal influenza vaccine

Broad neutralization of H1 and H3 viruses by adjuvanted influenza HA stem vaccines in nonhuman primates

Universal influenza virus vaccines and therapeutic antibodies

Comparison of the Safety and Immunogenicity of a Novel Matrix-M-Adjuvanted Nanoparticle Influenza Vaccine with a Quadrivalent Seasonal Influenza Vaccine in Older Adults: A Randomized Controlled Trial. medRxiv

A plant-derived quadrivalent virus like particle influenza vaccine induces cross-reactive antibody and T cell response in healthy adults

Traditional and new influenza vaccines

The evolving history of influenza viruses and influenza vaccines

Preventing an antigenically disruptive mutation in egg-based H3N2 seasonal influenza vaccines by mutational incompatibility

Low 2012-13 influenza vaccine effectiveness associated with mutation in the egg-adapted H3N2 vaccine strain not antigenic drift in circulating viruses

Effects of egg-adaptation on receptor-binding and antigenic properties of recent influenza A (H3N2) vaccine viruses

Immunogenicity and reactogenicity of influenza subunit vaccines produced in MDCK cells or fertilized chicken eggs

Current and emerging cell culture manufacturing technologies for influenza vaccines

The human cell line PER. C6 provides a new manufacturing system for the production of influenza vaccines

Development of a mammalian cell (Vero) derived candidate influenza virus vaccine

Evaluation of the safety, reactogenicity and immunogenicity of FluBlok® trivalent recombinant baculovirus-expressed hemagglutinin influenza vaccine administered intramuscularly to healthy children aged 6-59 months

Safety and immunogenicity of a virus-like particle pandemic influenza A (H1N1) 2009 vaccine in a blinded, randomized, placebo-controlled trial of adults in Mexico

CD4+ T cells: differentiation and functions

Pathways of antigen processing

Cellular and Molecular Immunology

Design of nanomaterial based systems for novel vaccine development

Cross-presentation by dendritic cells

Influenza vaccination strategies: comparing inactivated and live attenuated influenza vaccines

The role of serum haemagglutination-inhibiting antibody in protection against challenge infection with influenza A2 and B viruses

Immunogenicity and protective efficacy of influenza vaccination

Safety, immunogenicity and efficacy of intranasal, live attenuated influenza vaccine

Live attenuated influenza vaccine (LAIV) impacts innate and adaptive immune responses

Live Attenuated Influenza Vaccine [LAIV] (The Nasal Spray Flu Vaccine)

Avian flu: influenza virus receptors in the human airway

Vaccine adjuvants: putting innate immunity to work

Influenza pandemics of the 20th century

Adjuvant solution for pandemic influenza vaccine production

Factors affecting immune responses to the influenza vaccine

Overweight and obese adult humans have a defective cellular immune response to pandemic H1N1 influenza A virus

Immunogenicity and safety of influenza vaccination in systemic lupus erythematosus patients compared with healthy controls: a meta-analysis

Influenza vaccination in HIV-positive subjects: latest evidence and future perspective

The immunogenicity of influenza virus vaccine in solid organ transplant recipients

Adjuvanted influenza vaccines. Hum Vaccines Immunother

The history of MF59® adjuvant: a phoenix that arose from the ashes

Adjuvant systems for vaccines: 13 years of post-licensure experience in diverse populations have progressed the way adjuvanted vaccine safety is investigated and understood

Immunology and efficacy of MF59-adjuvanted vaccines. Hum Vaccines Immunother

Aluminum salts as an adjuvant for pre-pandemic influenza vaccines: a meta-analysis

B-cell responses to vaccination at the extremes of age

Towards an understanding of the adjuvant action of aluminium

Rate of disappearance of diphtheria toxoid injected into rabbits and guinea pigs: toxoid precipitated with alum

Aluminium hydroxide granuloma

Alum adjuvant boosts adaptive immunity by inducing uric acid and activating inflammatory dendritic cells

Vaccine adjuvants alum and MF59 induce rapid recruitment of neutrophils and monocytes that participate in antigen transport to draining lymph nodes

Crucial role for the Nalp3 inflammasome in the immunostimulatory properties of aluminium adjuvants

Aluminum hydroxide adjuvants activate caspase-1 and induce IL-1β and IL-18 release

The Nlrp3 inflammasome is critical for aluminium hydroxide-mediated IL-1β secretion but dispensable for adjuvant activity

Silica crystals and aluminum salts activate the NALP3 inflammasome through phagosomal destabilization

Local tissue irritating effects and adjuvant activities of calcium phosphate and aluminium hydroxide with different physical properties

Silica crystals and aluminum salts regulate the production of prostaglandin in macrophages via NALP3 inflammasome-independent mechanisms

MF59 design and evaluation of a safe and potent adjuvant for human vaccines

Distribution of adjuvant MF59 and antigen gD2 after intramuscular injection in mice

Enhancement of humoral response against human influenza vaccine with the simple submicron oil/water emulsion adjuvant MF59

The adjuvants aluminum hydroxide and MF59 induce monocyte and granulocyte chemoattractants and enhance monocyte differentiation toward dendritic cells

Dendritic cells internalize vaccine adjuvant after intramuscular injection

MF59 is a safe and potent vaccine adjuvant for flu vaccines in humans: what did we learn during its development?

Inflammasome-independent role of the apoptosisassociated speck-like protein containing CARD (ASC) in the adjuvant effect of MF59

Adjuvant formulations possess differing efficacy in the potentiation of antibody and cell mediated responses to a human malaria vaccine under selective immune genes knockout environment

The adjuvant MF59 induces ATP release from muscle that potentiates response to vaccination

Vitamin E: function and metabolism

Age-associated changes in immune and inflammatory responses: impact of vitamin E intervention

Adjuvant System AS03 containing α-tocopherol modulates innate immune response and leads to improved adaptive immunity

NF-κB and the innate immune response

Development and evaluation of AS03, an Adjuvant System containing α-tocopherol and squalene in an oil-in-water emulsion

Efficacy, immunogenicity, and safety of an oral influenza vaccine: a placebo-controlled and active-controlled phase 2 human challenge study

The adjuvant GLA-AF enhances human intradermal vaccine responses

A phase I randomized, double-blind, controlled trial of 2009 influenza A (H1N1) inactivated monovalent vaccines with different adjuvant systems. Vaccine

Development of VAX128, a recombinant hemagglutinin (HA) influenza-flagellin fusion vaccine with improved safety and immune response

Topical imiquimod before intradermal trivalent influenza vaccine for protection against heterologous non-vaccine and antigenically drifted viruses: a singlecentre, double-blind, randomised, controlled phase 2b/3 trial

Safety and immunogenicity of CPG 7909 injection as an adjuvant to Fluarix influenza vaccine

Evaluation of a virosomal H5N1 vaccine formulated with Matrix M™ adjuvant in a phase I clinical trial

Vaccination with a recombinant H7 hemagglutinin-based influenza virus vaccine induces broadly reactive antibodies in humans

Efficacy of FLU-v, a broad-spectrum influenza vaccine, in a randomized phase IIb human influenza challenge study

Safety and immunogenicity of a novel nanoemulsion mucosal adjuvant W805EC combined with approved seasonal influenza antigens

A phase I randomized, observer-blind, controlled, dose escalation trial of the safety and tolerability of a single intramuscular dose of a PAL adjuvant (laboratory code, FB-631) Co-administered with seasonal TIV (2013-2014) to healthy Adults ≥18-50 Years of age

Safety and immunogenicity of an influenza vaccine A/H5N1 (A/Vietnam/1194/ 2004) when coadministered with a heat-labile enterotoxin (LT) adjuvant patch

A double-blind, randomized controlled trial to evaluate the safety and immunogenicity of an intranasally administered trivalent inactivated influenza vaccine with the adjuvant LTh (αK): a phase II study

Randomized clinical trial of immunogenicity and safety of a recombinant H1N1/2009 pandemic influenza vaccine containing Advax™ polysaccharide adjuvant

Contrasting effects of type I interferon as a mucosal adjuvant for influenza vaccine in mice and humans

Toll-like receptor signaling pathways

Inactivated influenza vaccine formulated with single-stranded RNA-based adjuvant confers mucosal immunity and cross-protection against influenza virus infection

Prevention of serious events in adults 65 years of age or older: a comparison between high-dose and standarddose inactivated influenza vaccines

Safety and immunogenicity of a full-dose, splitvirion, inactivated, quadrivalent influenza vaccine in healthy children 6-35 months of age: a randomized controlled clinical trial

The comparative effectiveness of adjuvanted and unadjuvanted trivalent inactivated influenza vaccine (TIV) in the elderly

Targeting skin dendritic cells to improve intradermal vaccination

Fluzone® Intradermal vaccine: a promising new chance to increase the acceptability of influenza vaccination in adults

Flu Vaccine Safety Information

Adverse events of interest vary by influenza vaccine type and brand: sentinel network study of eight seasons

AS03 adjuvanted AH1N1 vaccine associated with an abrupt increase in the incidence of childhood narcolepsy in Finland

Risk of narcolepsy in children and young people receiving AS03 adjuvanted pandemic

influenza vaccine: retrospective analysis

Narcolepsy associated with Pandemrix vaccine

Autoantibodies in Pandemrix®-induced narcolepsy: nine candidate autoantigens fail the conformational autoantibody test

Role of mucosal immunity in influenza

Current and next generation influenza vaccines: formulation and production strategies

Covid-19: moderna vaccine is nearly 95% effective, trial involving high risk and elderly people shows

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

Nucleoside-modified mRNA immunization elicits influenza virus hemagglutinin stalkspecific antibodies

mRNA vaccines against H10N8 and H7N9 influenza viruses of pandemic potential are immunogenic and well tolerated in healthy adults in phase 1 randomized clinical trials

Influenza vaccines: Past, present, and future