Evaluation Of Safety,immunogenicity And Alternative Methods For Antibody Detection Of An Inactivated SARS-CoV-2 Vaccine In Healthy Adults Aged 18-59 Years

Objectives:(1)This study aimed to assess the safety and immunogenicity of an inactivated SARS-CoV-2 vaccine candidate in healthy Chinese adults 18-59 years of age.(2)In participants who received inactivated SARS-CoV-2 vaccines or placebo,specific antibody responses against the receptor binding domain(RBD)or N-terminal domain(NTD)were detected by Enzyme Linked Immunosorbent Assay(ELISA),and the antibody inhibition rates were measured by Homogeneous Time Resolved Fluorescence(HTRF).The results of these assays were compared with the neutralizing antibody titers to live SARS-CoV-2 to explore the feasibility of replacing the neutralizing antibody test to live SARS-CoV-2 with EILSA or HTRF.Methods: This single-center,randomized,double-blind,placebo-controlled,dose-escalating,phase 1 clinical trial was done in Jiangsu Province,China.Healthy adults 18-59 years of age were sequentially recruited(1:1)into two vaccination schedule cohorts,the days 0 and 14 or 0and 28 vaccination cohort.Each vaccination cohort was divided into two blocks: the first 36 participants were assigned to low dose group(3 μg per 0.5 ml),and the later 36 participants were assigned to high dose group(6 μg per 0.5 ml).Participants in each block were randomly assigned 2:1 to receive two doses of SARS-CoV-2 inactivated vaccine or placebo by intramuscular injection.The primary safety endpoint was the incidence of adverse reactions within 28 days post-vaccination in participants receiving at least one dose of study vaccine.The primary immunogenicity endpoints were the geometric mean titer(GMT)and seroconversion rates of neutralizing antibodies to live SARS-CoV-2 at days 14 and 28 after two-dose vaccination schedule.Specific antibody responses against the RBD or NTD were detected by ELISA,and the antibody inhibition rates were measured by HTRF.Correlation analysis of the RBD-binding or NTD-binding specific ELISA antibodies,the antibody inhibition rates and the neutralizing antibody titers to live SARS-CoV-2 was performed to assess the relationship between immune responses on different assays.This study is registered with Clinical Trials.gov,NCT04352608.Results:(1)185 volunteers 18-59 years of age were screened for eligibility,and 144 participants were eventually recruited in this phase 1 clinical trial.Of them,one participant assigned to placebo group in the days 0 and 28 cohort whithdrew before receiving the first dose of the vaccine.A total of 143 particiants completed the safety observation and immunogenicity evaluation for 28 days after two-dose vaccination schedule.72 participants(24 participants in each group)received the study vaccine or placebo in the days 0 and 14 vaccination cohort,and71 participants(24 participants in the 3 μg and 6 μg groups,and 23 participants in the placebo group)received the study vaccine or placebo in the days 0 and 28 vaccination cohort.(2)In the days 0 and 14 vaccination cohort,the incidence of adverse reactions within 28 days postvaccination in the 3 μg,6 μg,and placebo group were 29.2%(n=7),37.5%(n=9),and 8.3%(n=2),respectively;In the days 0 and 28 vaccination cohort,the incidence of adverse reactions within 28 days post-vaccination in the 3 μg,6 μg,and placebo group were 12.5%(n=3),16.7%(n=4),and 13.0%(n=3),respectively.The incidence of adverse reactions between 3 μg and 6μg groups were both no statistical difference in two vaccination schedule cohorts.The most commonly reported injection site adverse reaction was pain,and the most common systematic adverse reaction was fatigue.Most adverse reactions reported in all participants were mild in severity.No serious adverse event was observed within 28 days after each dose of vaccination.(3)Neutralizing antibody responses could be quickly induced at day 14 after the second dose of an inactivated SARS-CoV-2 vaccine.In the days 0 and 14 vaccination cohort,the seroconversion rates of neutralizing antibodies to live SARS-CoV-2 in the 3 μg,6 μg,and placebo group were 45.8%(n=11),50.0%(n=12),and 0.0%(n=0),respectively;In the days 0and 28 vaccination cohort,the seroconversion rates of neutralizing antibodies to live SARSCoV-2 in the 3 μg,6 μg,and placebo group were 79.2%(n=19),83.3%(n=20),and 0.0%(n=0),respectively.The neutralizing antibodies induced by 3 μg and 6 μg of vaccine were similar.(4)Regarding the neutralizing antibody test to live SARS-CoV-2 as a gold standard,the sensitivity(90.9% and 87.0%)and specificity(78.8% and 87.9%)of ELISA serial test and HTRF were high.The RBD-binding or NTD-binding specific ELISA antibodies and the antibody inhibition rates using HTRF were positively correlated with the neutralizing antibody titers to live SARSCoV-2 at days 14,28 after two-dose vaccination schedule,regardless of the dose groups and vaccination schedule,with a correlation coefficient of 0.84(RBD-binding specific ELISA antibodies),0.84(NTD-binding specific ELISA antibodies)and 0.86(antibody inhibition rates)(all P value < 0.0001),respectively.Conclusions: The inactivated SARS-CoV-2 vaccine is safe and immunogenic in healthy adults18-59 years of age.The humoral immune responses could be produced rapidly at day 14 after receiving two doses of vaccine,and the neutralizing antibodies induced by 3 μg and 6 μg of vaccine were similar.Our findings show high sensitivity and specificity of ELISA serial test and HTRF.In addition,the strong correlation between the RBD-binding or NTD-binding specific ELISA antibodies,the antibody inhibition rates and neutralizing antibody titers to live SARS-CoV-2 was found in this study,which make them possible to replace the neutralizing antibody tests to live SARS-CoV-2 in the future and are expected to be widely used in the phase3 clinical trials.