key: cord-0769217-29tl1pgf
authors: Liu, Jianyang; Mao, Qunying; Wu, Xing; He, Qian; Bian, Lianlian; Bai, Yu; Wang, Zhongfang; Wang, Qian; Zhang, Jialu; Liang, Zhenglun; Xu, Miao
title: Considerations for the Feasibility of Neutralizing Antibodies as a Surrogate Endpoint for COVID-19 Vaccines
date: 2022-04-27
journal: Front Immunol
DOI: 10.3389/fimmu.2022.814365
sha: 97f910bdd29e4b8bdafcea9773c22b6d4be28041
doc_id: 769217
cord_uid: 29tl1pgf

To effectively control and prevent the pandemic of coronavirus disease 2019 (COVID-19), suitable vaccines have been researched and developed rapidly. Currently, 31 COVID-19 vaccines have been approved for emergency use or authorized for conditional marketing, with more than 9.3 billion doses of vaccines being administered globally. However, the continuous emergence of variants with high transmissibility and an ability to escape the immune responses elicited by vaccines poses severe challenges to the effectiveness of approved vaccines. Hundreds of new COVID-19 vaccines based on different technology platforms are in need of a quick evaluation for their efficiencies. Selection and enrollment of a suitable sample of population for conducting these clinical trials is often challenging because the pandemic so widespread and also due to large scale vaccination. To overcome these hurdles, methods of evaluation of vaccine efficiency based on establishment of surrogate endpoints could expedite the further research and development of vaccines. In this review, we have summarized the studies on neutralizing antibody responses and effectiveness of the various COVID-19 vaccines. Using this data we have analyzed the feasibility of establishing surrogate endpoints for evaluating the efficacy of vaccines based on neutralizing antibody titers. The considerations discussed here open up new avenues for devising novel approaches and strategies for the research and develop as well as application of COVID-19 vaccines.

The current coronavirus disease 2019 pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS−CoV−2) is a disaster of unprecedented magnitude in modern times. On the other hand, the rapid research and development (R&D) and application of COVID-19 vaccines in response to the pandemic can be regarded as a miracle in the history of vaccine development. Over a span of less than 2 years, a total of 31 different types of COVID-19 vaccines around the world have been granted Emergency Use Authorizations (EUAs) or marketing approvals (1-3). As of January 15, 2022, more than 9.3 billion doses of COVID-19 vaccines have been administered worldwide (4) , but vaccination rates remain low in many regions and countries. The current lots of vaccines are far from perfect. Reduction in immunity over a period of time and lower efficiency against variants seem to be the major concerns. However, vaccines do offer a means to combat the pandemic. The R&D and application of vaccines with superior immunogenicity and universal effectiveness against all SARS− CoV−2 variants, are of utmost priority in addition to increasing vaccination coverage and administrating of a third (booster) COVID-19 vaccine dose (5) (6) (7) .

Phase III clinical trials are the main rate-limiting steps in vaccine R&D and application. In countries with high COVID-19 vaccination coverage or viral prevalence, it is difficult to conduct trials on the clinical efficacy of vaccines. The search for methods to rapidly and effectively evaluate vaccine's effectiveness has, therefore, become a bottleneck in subsequent vaccine R&D efforts. Surrogate endpoints may effectively save clinical time and are compliant with ethical standards. There has been a successful history of using antibodies as surrogate endpoints for other licensed viral vaccines. The key to establishing surrogate endpoints relies on finding a correlationship between vaccineinduced immune responses and the level of protection (8) . The cellular and humoral immunity induced by the vaccine synergistically protects the human body from viral infection (9, 10) . Antibodies, especially neutralizing antibodies, are key immunological markers that signal the elicitation of defense responses for the prevention and control of viral infections and disease onset. Consequently, the respective protective antibody levels have been used as surrogate endpoints for many viral vaccines such as influenza virus vaccine, measles vaccine, Japanese encephalitis vaccine, rabies vaccine, polio virus vaccine, hepatitis A vaccine, enterovirus 71 (EV71) vaccine, varicella vaccine and hepatitis B vaccine (Table 1 ) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) . In the case of measles, a pre-exposure neutralizing antibodies level in serum samples was positively correlated with clinical protection. Based on this finding, neutralizing antibodies titers of >120 mIU/mL were considered as a reliable surrogate endpoint for measles vaccine (12, 22) . Similarly, a titer of ≥ 20 mIU/mL was defined as seroconversion level for hepatitis A vaccine. This seroconversion rate showed a high level of agreement with clinical efficacy data of hepatitis A vaccine. Subsequently, in 2012, hepatitis A virus (HAV) IgG ≥ 20 mIU/ mL was included as the antibody threshold level for clinical effectiveness in the World Health Organization (WHO) technical reports on hepatitis A vaccines (17, (23) (24) (25) . A neutralizing antibody titer of 1:32 for EV71 vaccine has been recommended as the immunological surrogate endpoint because of its association with protection against EV71 (18, 19) . Previous clinical studies have confirmed that COVID-19 vaccineinduced humoral immunity generates effective neutralization antibodies against SARS-CoV-2 (26, 27) . The studies by Khoury et al. and Earle et al. demonstrated that there is a correlation between the level of neutralizing antibodies responses to SARS-CoV-2 and the protection level of the vaccine, which raises the possibility for the establishment of surrogate endpoint (28, 29) . T cell response is essential in inducing high-affinity antibodies and immune memory (30) . SARS-CoV-2 specific responsive T cell numbers are associated with protection against COVID-19 and accelerated viral clearance (31, 32) . In fact, mRNA vaccination induced early CD4 + T cell responses have been shown to correlate well with long-term humoral immunity. Robust cellular immune memory to SARS-CoV-2 and its variants persist for at least 6 months after mRNA vaccination (33) . However, the correlation between the T cell immunity and the protection level of COVID-19 vaccines is not very clear. In addition, compared to T cell immunity, antibody threshold levels are commonly used as surrogate endpoints for viral vaccines due to the ease of establishment of standardized test methods. At present, a large number of ongoing clinical trials of COVID-19 vaccines have not reported the threshold antibody levels of protection of their respective study populations. In addition, a lack of standardized neutralizing antibody detection methods and the effects of variants on the serum neutralizing activity of vaccines also pose challenges to the establishment of surrogate endpoints (5) . This present paper provides a review of the current status of research on neutralizing antibody responses and their utility in gauging the effectiveness of the various COVID-19 vaccines. An analysis of the feasibility of establishing surrogate endpoints based on neutralizing antibody levels in the hope of opening up new horizons for the R&D of an efficient and expedited application of COVID-19 vaccines.

The R&D and application of COVID-19 vaccines have progressed at an unparalleled pace in a global effort to control the ongoing pandemic. Figure 1A shows the major milestones in COVID-19 vaccine R&D and application. In December 2020, merely 1 year after the first report of COVID-19, the BNT162b2 vaccine jointly developed by Pfizer and BioNTech was granted the first-ever EUA in the United Kingdom (UK) (34) . This marked the start of large-scale, rapid vaccinations around the world ( Figure 1C) . Currently, 31 vaccines have received EUAs or conditional marketing authorizations, including two mRNA, eleven inactivated viruses, five adenoviral vectors, twelve recombinant subunits, and one DNA vaccine (1-3). As of January 15, 2022, eight COVID-19 vaccines have been approved for emergency use by the World Health Organization; namely, BNT162b2, AZD1222, Ad26.COV2.S, mRNA-1273, BBIBP-CorV, Coronavac, NVX-CoV2373 and Covaxin (35, 36) .

At present, the percentage of fully vaccinated people who have received all recommended doses of a COVID-19 vaccine has exceeded 50% in the United States of America (USA), Canada, and many developed countries of the European Union. In the two most populous countries in the world (China and India), the number of COVID-19 vaccine doses administered has reached 2.93 billion and 1.56 billion, respectively, which jointly account for approximately half of the total doses administered globally (37, 38) . With rapid vaccination efforts, the number of new COVID-19 cases worldwide showed a significant decrease between January and March 2021 ( Figure 1B) . However, the emergence of the Alpha, Beta, Gamma, Delta and Omicron variants with high transmissibility and an immune evasion ability has resulted in new waves of the pandemic, with the fourth wave caused by the Omicron variant on the rapid upswing ( Figure 1B ). In particular, the highly divergent Omicron variant, carrying over 30 mutations in the spike protein, has a substantial growth advantage over previous variant and has been identified in 149 countries since November 2021 (39) (40) (41) . Neutralization titer against Omicron is significantly reduced in convalescent sera from previous SARS-CoV-2 patient, sera after vaccination and therapeutic monoclonal antibodies (42) (43) (44) . The combined effects of the spread of the Delta and Omicron variants and decrease in neutralizing antibody titers with time after vaccination, have led to an increase in breakthrough infection rates in the real world. Therefore, current vaccines are no longer capable of effectively preventing breakthrough infections and the transmission of variants. The goal of vaccination seems to have shifted from preventing disease onset to reducing the number of critically ill patients and deaths (38, (45) (46) (47) .

To cope with the aforementioned situation, booster vaccination has become the strategy of choice for certain countries. Studies have indicated that a 5-25-fold increase in neutralizing antibody titer could be achieved after the administration of a third vaccine dose, which indicates a significant increase in efficacy compared with two vaccine doses (48) (49) (50) (51) . Real-world data from Israel showed that 12 days after the booster dose, the rate of confirmed infection was lower in the booster group than in the non-booster group by a factor of 11.3 (95% confidence interval (CI): 10.4 to 12.3), and the rate of severe illness decreased by a factor of 19.5 (95% CI: 12.9 to 29.5) (9). On October 21, 2021, Pfizer and BioNTech announced the first phase 3 clinical trial data of a booster dose of their COVID-19 vaccine. When Delta is the prevalent strain, the protection rate of booster vaccine is as high as 95.6% (52) . Compared with prime COVID-19 vaccination, a homologous and heterologous booster dose elicits potent neutralization titers against Omicron variant, which increasing vaccine effectiveness (28, [53] [54] [55] [56] [57] . Currently, some countries, including Israel, the USA, the UK, Switzerland, Germany, and China, have already launched booster vaccination campaigns (58) (59) (60) . However, research data on booster vaccination are relatively scarce. In particular, the duration of retention of adequate neutralizing antibody levels and mechanisms of titer decline after the booster shot remain unclear. Importantly, the safety of this approach has also not been adequately demonstrated yet (61) . Considering that infectious diseases know no borders and the vaccination coverage rates worldwide remain low, the WHO has repeatedly called for developed countries to refrain from the widespread rollout of booster shots until the percentage of fully vaccinated people of other nations have been adequately increased because the vaccination of a high percentage of the world's population may serves as the most effective pandemic prevention and control strategy (62, 63) . On October 26, 2021, the WHO Emergency Committee acknowledged that the COVID-19 pandemic is far from over, calling for the development of vaccines, diagnostic tools and therapeutics for long-term control of the pandemic (7) . To curb the spread of variants, institutions and companies around the world have embarked on the R&D of multi-variant COVID-19 vaccines (64-66).

The establishment of immunological surrogate endpoints is aimed at finding relevant indicators of vaccine protection through animal or human challenge experiments, and then using clinical data to obtain the relationship between immune protection indicators and clinical protection through different data statistical analysis models. Such an exercise produces the initial surrogate endpoints. Neutralizing antibody levels are generally used as the primary surrogate endpoint of the immunological protection of viral vaccines. Results of animal challenge models, efficacy of monoclonal antibodies, data from clinical trials of different vaccines, and real-world data of COVID-19 vaccines in many countries, have demonstrated the existence of a certain correlation between neutralizing antibody levels and an vaccine's effectiveness (5, 28, (67) (68) (69) (70) (71) .

Data from preclinical nonhuman primates challenge studies investigating the correlation between neutralizing antibody levels and vaccine effectiveness have indicated that vaccines developed using various technologies are capable of inducing the production of neutralizing antibodies in nonhuman primates. The neutralizing antibody titers induced in nonhuman primates by the vast majority of vaccine candidates tested in Phase III clinical trials are within the range of 100-5000. Despite significant differences in neutralizing antibody levels, all of these vaccines are capable of reducing the pathological response and viral load in the bronchoalveolar lavage or lungs to a certain extent ( Table 2) . Studies on the effectiveness of adenoviral vector vaccines (Johnson & Johnson) (76) and DNA vaccines (80) have revealed that the viral load in lungs is negatively correlated with the neutralizing antibody titer (R values: −0.5714-−0.7702) and that the neutralizing antibody titer must not be lower than 100-250 for the vaccine in order to provide full protection.

In recent months, vaccine manufacturers have published phase III COVID-19 vaccine clinical trial data, which have indicated vaccine efficacies consistent with those reported in phase I/II clinical trials. Although differences exist among specific vaccines, the various vaccines have exhibited good effects in the prevention of disease onset and critical illness ( Table 3) . At present, data related to the correlation between efficacy and neutralizing antibody titer in phase III clinical trials have not yet been reported. A meta-analysis that compared the correlations between efficacy and neutralization titer in recovering subjects across several phase III clinical trials revealed the presence of a strong nonlinear relationship between mean neutralization level and efficacy, which is in agreement with results obtained from animal challenge models (28). However, inconsistencies in the selected serum samples of the recovery phase may reduce the credibility of this relationship. In addition, large differences exist in the neutralizing antibody titers induced by similarly efficacious vaccines developed by different manufacturers. This may be attributed to differences in the sample population, test methods, tested variants, and dominant variant in the country of residence of the subjects, among phase III clinical trials conducted by different vaccine manufacturers ( Table 3 ). In a recent study, Feng et al. analyzed the clinical data of the ChAdOx1 nCoV-19 (AZD1222) vaccine in the UK and found that the vaccine efficacy was associated with antibody levels (especially those of neutralizing antibodies). Measurements of neutralizing antibody titers revealed values of 938 international units (IU)/mL was associated with 90% VE against symptomatic infection at 28 days post-vaccination (71) . A study on breakthrough infections in vaccinated healthcare workers at the largest medical center in Israel predicted that breakthrough infections could only be effectively prevented when the geometric mean titer (GMT) exceeded 533.7 (95% CI: 408.1 to 698.0) (98).

With the publication of real-world data, it is apparent that current COVID-19 vaccines that have been granted EUAs or conditional marketing authorizations, achieve different efficacies in individuals of different age groups, ethnicities, and countries. However, all vaccines have met the minimum efficacy requirement of 50% set by the WHO for EUAs and demonstrated high efficacies against progression towards severe disease or death ( Table 4 ) (26, (99) (100) (101) (102) . The effectiveness of the vaccines is positively correlated with the level of neutralizing antibodies ( Table 3) , which is consistent with Knory's and Earle's studies (28, 29) . Although the efficacies of current vaccines against the SARS−CoV−2 variants of concern (VOC) have decreased significantly compared to the preclinical stage, these vaccines still provide certain protective effects, especially against critical illness and disease-related death (27, 102, 103) .

The decrease in the efficacy of current vaccines has mainly been caused by the following: (1) Neutralizing antibodies generated after vaccination with current vaccines providing weaker neutralizing effects against newly emerged variants, resulting in (106) (107) (108) . Studies have found that booster vaccination leads to increased neutralizing antibody levels against variants and enhances vaccine efficacy (49, 50, 52) . Both clinical and realworld data demonstrate the presence of a positive correlation between neutralizing antibody level and vaccine effectiveness, 

*Alpha or Delta variant was highly prevalent in this region in this study. which provides a scientific basis for further data collection and analysis, to validate the use of neutralizing antibody threshold values as surrogate endpoints.

In recent clinical studies of two COVID-19 vaccines that were performed around the world, neutralizing antibody immune bridging was used to assess vaccine's effectiveness. The two coprimary endpoint criteria used were: 1. The neutralizing antibody titer is higher compared to active comparator vaccine AZD1222 (ChAdOx1-S); 2. The seroconversion rate of neutralizing antibody is more than the set threshold of 50% or 95% (109, 110) . After full immunization schedule, seroconversion of SARS-CoV-2-specific neutralizing antibodies is defined as 4-fold increase from baseline in the phase III clinical of VLA2011 (111 , and the neutralizing antibody seroconversion rate is more than 95% (110, 115) . The efficacies of MVC-COV1901 and VLA2001 vaccines will be evaluated based on real-world vaccination data. This will be the direct and effective verification of the feasibility of using neutralizing antibodies as surrogate endpoints for COVID-19 vaccines.

Neutralizing antibody testing can be performed using live virus, pseudovirus neutralization assays and lateral flow immunoassay (116) . A report by the WHO revealed the presence of differences in the experimental methods, variants, and calculation methods used for neutralizing antibody testing among different laboratories around the world, which resulted in significant biases in the measured neutralizing antibody levels of the same sample. were adopted (117) . Results indicated that with the exception of two low-titer samples and one negative sample for which the titers could not be easily calculated, the total GMTs of seven collaborative calibration samples determined by Live-PRNT, Live-FRNT, Live-CPE, and Live-MN were 317.1, 445.3, 93.9, and 239.6, respectively. When the same assay was used, the average fold (the ratio of the maximum value to the minimum value) in GMT across different laboratories were 14.7-, 12.9-, 19.5-, and 4.5-fold, respectively. Total GMTs determined by PV-LVV and PV-VSV were 371.8 and 519.2, respectively; average fold in GMT across different laboratories when the same method was used were 908.7-and 10.9-fold, respectively. During the collaborative calibration of the first Chinese national standards for SARS-CoV-2 neutralizing antibody, live virus neutralization assays performed by four laboratories (including Live-PRNT and Live-CPE), with one laboratory adopting two types of assays for testing and the remaining three laboratories using the CPE assay, and pseudovirus neutralization assays performed by 10 laboratories (all PV-VSV) were adopted. As shown in Table 5 , the GMTs of four collaborative calibration samples determined by different assays are significantly inconsistent. When the same assay was used, the fold (the ratio of the maximum value to the minimum value) in GMT across different laboratories are more than 4.7 at least. These results indicate the presence of considerable differences among different laboratories and methods ( Studies have shown that differences in neutralizing antibody test results among different laboratories were significantly decreased with the use of the WHO International Standard and the Chinese National Standards (117, 118) . When the International Standard was adopted, the total geometric coefficients of variation (GCVs) of five high-and medium-titer samples were reduced from 249%, 179%, 231%, 281%, and 161% to 94%, 95%, 119%, 67%, and 93%, and the upper quartile/lower quartile (UQ/LQ) values were reduced by 1.432-, 1.978-, 2.206-, 6.511-, and 2.348-fold. However, a low-titer sample did not exhibit a significant decrease in GCV and only showed a 0.97fold decrease in UQ/LQ compared with the pre-standardization value, which may be related to the fact that the low neutralizing antibody titer was close to the threshold value (117) . In the collaborative calibration of the Chinese national standards, the GCV values of three collaborative calibration samples measured by different laboratories using an authentic virus neutralization assay were 129%, 266%, and 146%. When standards with a titer of 1000 U/mL were used, the total GCVs among laboratories were reduced to 107%, 18%, and 90% (118) .

The standardization of neutralizing antibody test methods directly affects the establishment of immunological surrogate endpoints and has become a major influencing factor of COVID-19 vaccine R&D and evaluation. With the establishment of the first standard pseudovirus neutralization assay by Chinese researchers (119) and WHO's subsequent approval of the First International Standard for anti-SARS-CoV-2 immunoglobulin (human), methods for measuring neutralizing antibody titer can be standardized and the comparability of cross-platform test results can be effectively enhanced (120) .

As trials on the clinical effectiveness of vaccines constitute the main rate-limiting step in vaccine R&D, the current status of COVID-19 vaccine application and R&D urgently require the establishment of immunogenicity surrogate endpoints for testing vaccine's effectiveness. Recently, a number of reports on clinical and real-word effectiveness of COVID-19 vaccines have been published, and two COVID19 vaccines have reported positive comparative immunogenicity trial results (109) (110) (111) (112) (113) (114) . We are beginning to harness the potential utility of these data for establishing surrogate endpoints. However, the current guidance documents on immunological surrogate endpoints are not systematic and comprehensive. There are obstacles to sharing and analyzing large amounts of clinical data among vaccines manufactures. In addition, the lack of a standardized neutralizing antibody test assay and continuously emerging variants further complicate the comparisons. The robustness of immunological surrogate endpoints will largely depend on how we address these issues.

The pandemic of COVID-19 requires the WHO and national regulatory agencies to quickly assess the protective effectiveness of vaccines. Recently, the WHO and pharmaceutical regulatory agencies of the USA, UK, and China have indicated the need for studies investigating the relationships between vaccine-induced neutralizing antibody levels and vaccine's effectiveness ( Table 6 ). In particular, the Medicines and Healthcare products Regulatory Agency (MHRA) of the UK has announced that neutralizing antibody surrogate endpoints established using the WHO standard units can be applied to immunobridging studies in the R&D of vaccines against SARS−CoV−2 variants (121) (122) (123) (124) .

Guidelines issued by the Center for Drug Evaluation (CDE) of the National Medical Products Administration (NMPA) of China state that the correlation between immunological markers and protection should be investigated in COVID-19 vaccine clinical trials, and the establishment of surrogate endpoints requires the provision of evidence for five different aspects, which include the correlations of immune response mechanisms and antibody levels (particularly those of neutralizing antibodies) with protection (123) . Considering the fact that the current global outbreaks are caused by SARS−CoV−2 variants, the Unite States Food and Drug Administration (USFDA) has indicated the need to assess the neutralization of the virus from which the prototype vaccine was derived, as well as variants of concern with clinical serology samples obtained from persons immunized with vaccines against variants, when investigating the effectiveness of newly developed vaccines against variants (124) . Guidance provided by the WHO recommends the approach of looking for concordance of neutralization data and vaccine effectiveness results for VOCs to estimate the effectiveness of vaccines against new variants (121) . The MHRA considers that the weight of evidence from studies with authorized vaccines is sufficient to support the use of neutralizing antibody titers as a primary endpoint in cross-platform immunobridging trials. Therefore, neutralizing antibody titers may be justified as an immune marker to predict vaccine effectiveness, but they should be standardized using the WHO reference standards and expressed in terms of IU (125) . To guide deliberations regarding vaccines against variants, the European Medicines Agency (EMA) requires the evaluation of the neutralizing antibody levels elicited by the vaccines against variants and the parent strain under standardized conditions to serve as a secondary endpoint (122) .

Currently, 31 different types of COVID-19 vaccines have received EUAs or conditional marketing authorizations, and preclinical, clinical, and real-world data on efficacy and neutralizing antibody levels are available for many of these vaccines. However, inadequacies exist in the openness and correlation analyses of the efficacy and neutralizing antibody data of current vaccines (28, 71).The clinical studies of MVC-COV1901 and VLA2001 vaccines provide the results of immune bridging of neutralizing antibodies, but there is a lack of data on the protection rate of vaccines. These data could be jointly analyzed by vaccine manufacturers and regulatory agencies, and coordination efforts could be made by international health organizations for the standardized formulation of scientific surrogate endpoints. These will be greatly beneficial to the screening of high-immunogenicity vaccines from the immense number of vaccines being subjected to preclinical and clinical testing worldwide, maximization of the protection of participants and saving various resources. On the basis of existing clinical data, statistical tools may be utilized to investigate the relationships between neutralizing antibody level and vaccine efficacy, changes in neutralizing antibody level with time, and threshold levels of neutralizing antibody protection against different variants (28, 71). However, neutralizing antibodies are likely not the only mechanism of protection (126) . Further clinical and real-world data are required to determine the ability of neutralizing antibody levels to accurately evaluate the effectiveness of COVID-19 vaccines when used as surrogate endpoints.

There are a number of neutralizing antibody test methods available for the estimation of antibodies against SARS-CoV2 (116) . Standardized laboratory serological test methods are a prerequisite for the realization of data comparability among different laboratories and platforms for surrogate endpoint establishment. The standardization of test methods has become the main rate-limiting step in the R&D of COVID-19 vaccines. On the basis of the WHO antibody standard, countries and regions are advised to expedite the establishment of secondary national standards to specify requirements for the use of IU in Based on the specifics of the product under consideration, a neutralizing antibody titer may be justified as an immune marker to predict vaccine effectiveness. However, neutralizing antibody titers should be determined using the WHO-certified reference standards. 2. The weight of evidence from studies with authorized COVID-19 vaccines is sufficient to support the use of a neutralizing antibody titer as a primary endpoint in cross-platform immunobridging trials. (125) test methods. This will enable the comparison of neutralizing antibody test results across different studies for the accurate analysis and determination of correlations between neutralizing antibody levels and vaccine efficiencies.

SARS−CoV−2 is highly prone to mutations. To date, several hundreds of lineages have already emerged and new lineages are still appearing on a continuous basis. In particular, lineages with immune evasion abilities exert greater effects on vaccine protection (127) . Serum neutralizing capacity has been used as an efficient indicator to quickly assess the protective effect of vaccines on emerging variants. Results of these studies show that compared with the early strains, the neutralizing antibody titer against new variants has been significantly decreased (5, 128, 129) . Surrogate endpoints established using current clinical data will inevitably face challenges posed by continuously emerging variants. Therefore, continuous endpoint revisions may be required based on changes in variant dominance with time and the R&D status of vaccines (130) .

In addition, SARS-CoV-2 specific T cell immunity acquired by COVID-19 vaccines or previous infection still remain broadly robust and long-term protection against VOCs, including Omicron variant (131) (132) (133) (134) . A standardized measurement of T cells immunity may be served as an potential surrogate endpoint to better assess the protective effect of COVID-19 vaccines on emerging variants subsequently (135) .

The global scale of the pandemic, high vaccination coverage rates and ethical requirements, pose challenges to the effectiveness of subsequently developed COVID-19 vaccines in clinical trials. The establishment of immunological surrogate endpoints is of great significance to the acceleration of efficacy evaluations of vaccines. Current studies have indicated that vaccineinduced neutralizing antibody levels are correlated with clinical protection, and predictable clinical progression have tried to assess clinical protection through neutralizing antibody immunobridging experiments (109, 110) . Research efforts on surrogate endpoints should focus on the establishment and application of standardized test methods, and adoption of the WHO international standard to express titers in terms of IU and reduce measurement errors. In addition, it should also analyze the threshold levels of neutralization antibody protection against different variants based on current clinical data, and appropriate endpoint adjustments based on changes in variant circulation and vaccine R&D. Due to the rapid waning of neutralization tier, it is critial to assess quality and durability of the neutralization antibody in conjunction with standardization of time lines after vaccination (28). During the use of COVID-19 vaccines in clinical trials of booster and sequential vaccination, the threshold levels of new vaccines' neutralization antibody should be significantly superior to those of the primary vaccines. Higher titers of neutralization antibody will serve as an indication of the superior effectiveness of these new vaccines. 

Indicative Announcement on the Authorization of the Inactivated COVID-19 Vaccine for Emergency Use

Zydus Receives EUA From DCGI for ZyCoV-D, the Only Needle-Free COVID Vaccine in the World

World Health Organization. WHO Coronavirus (COVID-19) Dashboard (2021)

Reduced Sensitivity of SARS-CoV-2 Variant Delta to Antibody Neutralization

How to Redesign COVID Vaccines So They Protect Against Variants

Available at: https:// www.who.int/news/item/26-10-2021-statement-on-the-ninth-meeting-ofthe-international-health-regulations-(2005)-emergency-committeeregarding-the-coronavirus-disease-(covid-19)-pandemic

A Correlate of Protection for SARS-CoV-2 Vaccines is Urgently Needed

Contributions of Humoral and Cellular Immunity to Vaccine-Induced Protection in Humans

Role of Humoral Versus Cellular Responses Induced by a Protective Dengue Vaccine Candidate

World Health Organization. Recommendations for the Production and Control of Influenza Vaccine (Inactivated)

WHO Position Papers on Measles

Safety and Efficacy of Japanese Encephalitis Vaccines (Live, Attenuated) for Human Use

Recommendations for Inactivated Rabies Vaccine for Human Use Produced in Cell Substrates and Embryonated Eggs

Safety and Efficacy of Poliomyelitis Vaccines (Oral, Live, Attenuated)

Recommendations to Assure the Quality, Safety and Efficacy of Poliomyelitis Vaccines (Inactivated)

WHO Position Paper on Hepatitis A Vaccines

Efficacy, Safety, and Immunology of an Inactivated Alum-Adjuvant Enterovirus 71 Vaccine in Children in China: A Multicentre, Randomised, Double-Blind, Placebo-Controlled, Phase 3 Trial

Safety, and Immunogenicity of an Enterovirus 71 Vaccine in China

Inverse Relationship Between Six Week Postvaccination Varicella Antibody Response to Vaccine and Likelihood of Long Term Breakthrough Infection

Recommendations to Assure the Quality, Safety and Efficacy of Recombinant Hepatitis B Vaccines. Annex 4 of WHO TRS No. 978

Measles Antibody: Reevaluation of Protective Titers

Serum Neutralizing Antibody Response to Hepatitis A Virus

Clinical Assessment of the Safety and Efficacy of an Inactivated Hepatitis A Vaccine: Rationale and Summary of Findings

Clinical Experience With an Inactivated Hepatitis A Vaccine

Effectiveness of an Inactivated SARS-CoV-2 Vaccine in Chile

mRNA-1273 COVID-19 Vaccine Effectiveness Against the B.1.1.7 and B.1.351 Variants and Severe COVID-19 Disease in Qatar

Evidence for Antibody as a Protective Correlate for COVID-19 Vaccines

Antibodies and B Cell Memory in Viral Immunity

SARS-CoV-2 Responsive T Cell Numbers and Anti-Spike IgG Levels are Both Associated With Protection From COVID-19: A Prospective Cohort Study in Keyworkers. medRxiv (2021) 2020

Early Induction of Functional SARS-CoV-2-Specific T Cells Associates With Rapid Viral Clearance and Mild Disease in COVID-19 Patients

mRNA Vaccines Induce Durable Immune Memory to SARS-CoV-2 and Variants of Concern

Vaccine BNT162b2 -Conditions of Authorisation Under Regulation 174

FDA Approves First COVID-19 Vaccine

Data. COVID-19 Vaccine Doses Administered

National Health Commission of the Peoples's Republic of China

Rapid Epidemic Expansion of the SARS-CoV-2 Omicron Variant in Southern Africa

529)-technical-brief-and-priorityactions-for-member-states

Potential Rapid Increase of Omicron Variant Infections in the United States

SARS-CoV-2 Omicron Variant Escapes Neutralization by Vaccinated and Convalescent Sera and Therapeutic Monoclonal Antibodies. medRxiv (2021) 2021

An Infectious SARS-CoV-2 B.1.1.529 Omicron Virus Escapes Neutralization by Several Therapeutic Monoclonal Antibodies

Considerable Escape of SARS-CoV-2 Omicron to Antibody Neutralization

AZD1222-Induced Neutralising Antibody Activity Against SARS-CoV-2 Delta VOC

Prevention and Attenuation of Covid-19 With the BNT162b2 and mRNA-1273 Vaccines

Effectiveness of COVID-19 Vaccines Against the Omicron (B.1.1.529) Variant of Concern

A Third Dose of Inactivated Vaccine Augments the Potency, Breadth, and Duration of Anamnestic Responses Against SARS-CoV-2. medRxiv (2021) 2021

Protection of BNT162b2 Vaccine Booster Against Covid-19 in Israel

Centers for Disease Control and Prevention. Evidence to Recommendation Framework: Pfizer-BioNTech COVID-19 Booster Dose

SARS-CoV-2 Neutralization With BNT162b2 Vaccine Dose 3

And BioNTech Announce Phase 3 Trial Data Showing High Efficacy Of A Booster Dose Of Their COVID-19 Vaccine

Pseudotyped SARS-CoV-2 Omicron Variant Exhibits Significant Escape From Neutralization Induced by a Third Booster Dose of Vaccination. medRxiv (2021) 2021

Booster of mRNA-1273 Vaccine Reduces SARS-CoV-2 Omicron Escape From Neutralizing Antibodies. medRxiv (2021) 2021

mRNA-Based COVID-19 Vaccine Boosters Induce Neutralizing Immunity Against SARS-CoV-2 Omicron Variant

Third BNT162b2 Vaccination Neutralization of SARS-CoV-2 Omicron Infection

Plasma Neutralization of the SARS-CoV-2 Omicron Variant

FDA Authorizes Booster Dose of Pfizer-BioNTech COVID-19 Vaccine for Certain Populations

Data. COVID-19 Vaccine Booster Doses Administered

The State Council. The People's Republic of China (2021)

Considerations in Boosting COVID-19 Vaccine Immune Responses

WHO Reiterates Warning Against Covid Boosters for Healthy People as U.S. Weighs Wide Distribution of Third Shots

UN Set Out Steps to Meet World COVID Vaccination Targets

Available at

Reactogenicity and Immunogenicity Study of ReCOV

Gritstone Starts Phase I Dosing of Second-Gen mRNA Covid-19 Vaccine

SARS-CoV-2 Neutralizing Antibody LY-CoV555 in Outpatients With Covid-19

A Systematic Review of Antibody Mediated Immunity to Coronaviruses: Kinetics, Correlates of Protection, and Association With Severity

Correlates of Protection Against SARS-CoV-2 in Rhesus Macaques

s Neutralizing Antibody Bamlanivimab (LY-CoV555) Prevented COVID-19 at Nursing Homes in the BLAZE-2 Trial, Reducing Risk by Up to 80 Percent for Residents

Correlates of Protection Against Symptomatic and Asymptomatic SARS-CoV-2 Infection

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

A Prefusion SARS-CoV-2 Spike RNA Vaccine is Highly Immunogenic and Prevents Lung Infection in non-Human Primates

Immunogenicity and Efficacy of One and Two Doses of Ad26.COV2.S COVID Vaccine in Adult and Aged NHP

ChAdOx1 Ncov-19 Vaccination Prevents SARS-CoV-2 Pneumonia in Rhesus Macaques

ChAdOx1 Ncov-19 Vaccine Prevents SARS-CoV-2 Pneumonia in Rhesus Macaques

Development of an Inactivated Vaccine Candidate, BBIBP-CorV, With Potent Protection Against SARS-CoV-2

Development of an Inactivated Vaccine Candidate for SARS-CoV-2

Immunogenicity and Protective Efficacy of Inactivated SARS-CoV-2 Vaccine Candidate, BBV152 in Rhesus Macaques

One or Two Dose Regimen of the SARS-CoV-2 Synthetic DNA Vaccine INO-4800 Protects Against Respiratory Tract Disease Burden in Nonhuman Primate Challenge Model

NVX-CoV2373 Vaccine Protects Cynomolgus Macaque Upper and Lower Airways Against SARS-CoV-2 Challenge

COVID-19 Subunit Vaccine Candidate, Induces Protective Immunity in Nonhuman Primates

Effect of 2 Inactivated SARS-CoV-2 Vaccines on Symptomatic COVID-19 Infection in Adults: A Randomized Clinical Trial

Efficacy and Safety of a COVID-19 Inactivated Vaccine in Healthcare Professionals in Brazil: The PROFISCOV Study

Efficacy and Safety of an Inactivated Whole-Virion SARS-CoV-2 Vaccine (CoronaVac): Interim Results of a Double-Blind, Randomised, Placebo-Controlled, Phase 3 Trial in Turkey

Safety and Efficacy of the ChAdOx1 Ncov-19 Vaccine (AZD1222) Against SARS-CoV-2: An Interim Analysis of Four Randomised Controlled Trials in Brazil, South Africa, and the UK

Safety and Immunogenicity of the ChAdOx1 Ncov-19 Vaccine Against SARS-CoV-2: A Preliminary Report of a Phase 1/2, Single-Blind, Randomised Controlled Trial

Safety and Efficacy of an Rad26 and Rad5 Vector-Based Heterologous Prime-Boost COVID-19 Vaccine: An Interim Analysis of a Randomised Controlled Phase 3 Trial in Russia

Safety, and Lot to Lot Immunogenicity of an Inactivated SARS-CoV

Vaccine (BBV152): A Double-Blind, Randomised

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

An mRNA Vaccine Against SARS-CoV-2 -Preliminary Report

Safety and Efficacy of the BNT162b2 mRNA Covid-19 Vaccine

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

Safety and Efficacy of Single-Dose Ad26.Cov2.S Vaccine Against Covid-19

Interim Results of a Phase 1-2a Trial of Ad26.COV2.S Covid-19 Vaccine

Safety and Efficacy of NVX-CoV2373 Covid-19 Vaccine

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

Covid-19 Breakthrough Infections in Vaccinated Health Care Workers

Effectiveness of BNT162b2 and mRNA-1273 Covid-19 Vaccines Against Symptomatic SARS-CoV-2 Infection and Severe Covid-19 Outcomes in Ontario, Canada: Test Negative Design Study

BNT162b2 mRNA Covid-19 Vaccine in a Nationwide Mass Vaccination Setting

Butt AANational Study Group for COVID-19 Vaccination. Effectiveness of the BNT162b2 Covid-19 Vaccine Against the B.1.1.7 and B.1.351 Variants

Comparison of Two Highly-Effective mRNA Vaccines for COVID-19 During Periods of Alpha and Delta Variant Prevalence

Neutralisation of SARS-CoV-2 Lineage P.1 by Antibodies Elicited Through Natural SARS-CoV-2 Infection or Vaccination With an Inactivated SARS-CoV-2 Vaccine: An Immunological Study

Effectiveness of Covid-19 Vaccines Against the B.1.617.2 (Delta) Variant

Impact of Delta on Viral Burden and Vaccine Effectiveness Against New SARS-CoV-2 Infections in the UK. medRxiv (2021) 2021

Durability of Responses After SARS-CoV-2 mRNA-1273

Studies on the Level of Neutralizing Antibodies Produced by Inactivated COVID-19 Vaccines in the Real World. medRxiv (2021) 2021

Waning Immunity of the BNT162b2 Vaccine: A Nationwide Study From Israel

Medigen Vaccine Biologics Corp. MVC COVID-19 Vaccine Obtains Taiwan EUA Approval

Valneva Reports Positive Phase 3 Results for Inactivated, Adjuvanted COVID-19 Vaccine Candidate Vla2001 (2021)

Study To Compare The Immunogenicity Against COVID-19, Of VLA2001 Vaccine To AZD1222 Vaccine (COV-COMPARE) (2021)

Safety and Immunogenicity of CpG 1018 and Aluminium Hydroxide-Adjuvanted SARS-CoV-2 S-2P Protein Vaccine MVC-COV1901: Interim Results of a Large-Scale, Double-Blind, Randomised, Placebo-Controlled Phase 2 Trial in Taiwan

Safety and Immunogenicity of a Recombinant Stabilized Prefusion SARS

Spike Protein Vaccine (MVC COV1901) Adjuvanted With CpG 1018 and Aluminum Hydroxide in Healthy Adults: A Phase 1, Dose-Escalation Study. EClinicalMedicine

Reports Positive Phase 1/2 Data for Its Inactivated, Adjuvanted COVID-19 Vaccine Candidate

Recent Developments in SARS-CoV-2 Neutralizing Antibody Detection Methods

WHO/BS.2020.2403 Establishment of the WHO International Standard and Reference Panel for Anti-SARS-CoV-2 Antibody

The First Chinese National Standards for SARS-CoV-2 Neutralizing Antibody

Establishment and Validation of a Pseudovirus Neutralization Assay for SARS-CoV-2

WHO International Standard for Anti-SARS-CoV-2 Immunoglobulin

Guidance on Conducting Vaccine Effectiveness Evaluations in the Setting of New SARS-CoV-2 Variants

Regulatory Requirements for Vaccines Intended to Provide Protection Against Variant Strain(s) of SARS-CoV-2 (2021)

National Medical Products Administration. Technical Guidelines for the Development of Novel Coronavirus Preventive Vaccines (Trial

GUIDANCE DOCUMENT Emergency Use Authorization for Vaccines to Prevent COVID-19 Guidance for Industry

Medicines & Healthcare products Regulatory Agency. Decision Access Consortium: Alignment With ICMRA Consensus on Immunobridging for Authorising New COVID-19 Vaccines

COVID-19 Vaccines: Keeping Pace With SARS-CoV-2 Variants

SARS-CoV-2 Omicron: Evasion of Potent Humoral Responses and Resistance to Clinical Immunotherapeutics Relative to Viral Variants of Concern

Neutralization of Beta and Delta Variant With Sera of COVID-19 Recovered Cases and Vaccinees of Inactivated COVID-19 Vaccine BBV152/Covaxin

Superspreading in the Emergence of COVID-19 Variants

SARS-CoV-2 T Cell Responses Elicited by COVID-19 Vaccines or Infection Are Expected to Remain Robust Against Omicron

Impact of SARS-CoV-2 Variants on the Total CD4(+) and CD8(+) T Cell Reactivity in Infected or Vaccinated Individuals

Signature of Long-Lived Memory CD8+ T Cells in Acute SARS-CoV-2 Infection

Cross-Reactive Memory T Cells Associate With Protection Against SARS-CoV-2 Infection in COVID-19 Contacts

Correlates of Protection -Will Emerging Data Allow Increased Reliance on Vaccine Immune Responses for Public Health and Regulatory Decision-Making? (2022)

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.