Humoral immune response in inactivated SARS-CoV-2 vaccine: When should a booster dose be administered?

Kara, E.; Tanir, F.; Demirhindi, H.; Mete, B.; Kibar, F.; Cetiner, S.; Candevir, A.; Inaltekin, A.

Background: For a sustained and essential protective antibody response, it is important to understand how long the humoral immune response induced by the SARS-CoV-2 inactivated vaccine persists. Aims: This study aimed to detect the first and third-month concentrations and seroconversion rates of the antibodies induced by the inactivated vaccine. Study Design: This is a vaccine efficacy study. Methods: The study included 272 health workers who were vaccinated at days 0 and 28 by the inactivated SARS-CoV-2 vaccine (3{micro}g/0.5ml). Anti-S-RBD-IgG and total anti-spike/anti-nucleocapsid-IgG antibody concentrations and seroconversion rates were examined in vaccinated health workers at the 1st and 3rd months after the vaccination. The test method used for the qualitative detection and differentiation of IgG antibodies (indirect method) to SARS-CoV-2 is a chemiluminescence reaction (CLIA). Results: The mean age of the health workers was 38.93{+/-}10.59 (min:21-max:64). A total of 45(16.5%) participants declared to have had COVID-19 before the first dose of the inactivated vaccine. The participants were found to be reactive for anti-S-RBD-IgG antibodies by 98.2% and 97.8% at the first and third months, respectively, after the administration of the second dose. The decrease in the mean plasma concentrations of anti-S-RBD IgG was observed as 56.7% in the cohort with only two doses of the vaccine (1st month:42.4AU/ml versus 3rd month: 18.2AU/ml). In the cohort with a history of COVID-19 prior to the vaccination, the decrease was observed as 25.1% (1st month:58.29 versus 3rd month:43.64 AU/ml) and at a mean of 57.4 (0-90) days prior to vaccination, the decrease was of 43.1% (1st month:55.05 AU/ml versus 3rd month:31.28 AU/ml), keeping more stable in participants infected at a mean of 183.1 (91-330) days prior to vaccination (a decrease of 5.2%; with 62.34 AU/ml at 1st and 59.08 AU/ml at 3rd months). Anti-S-RBD concentrations were observed to increase 10-fold (30.44 AU/ml at 1st and 310.64 AU/ml at 3rd months) in participants infected after the vaccination and to decrease among people aged 50 years and older. Conclusion: Antibody concentrations at the 1st and 3rd months after the vaccination with two doses of the inactivated SARS-CoV-2 vaccine were found to be decreased, but still detectable (except in one participant). As participants who had COVID-19 at a mean of 181 (90-330) days before the vaccination presented with a more stable antibody level, it can be concluded that a booster at months 6-12, resulting in a schedule of 0-1-6 months, is recommended for the inactive SARS-CoV-2 vaccination. Keywords: Humoral immune response, vaccines, SARS-CoV-2, booster, inactive vaccine