[Humoral Immune Response in Inactivated SARS-CoV-2 Vaccine: When Should a Booster Dose be Administered?]

Vaccination induces the development of long-lasting immunity necessary for a sustained and essential protective antibody response. Understanding how long the humoral immune response induced by the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) inactivated vaccine lasts is important to create an effective immunization scheme. This study aimed to detect the first, third, sixth month concentrations and seroconversion rates of the antibodies induced by the inactivated vaccine. The study included 282 healthcare workers who were vaccinated at days 0 and 28 by the inactivated SARS-CoV-2 vaccine (3 μg/0.5ml). Anti-S-RBD-IgG (receptor binding domain) antibody concentrations and seroconversion rates were examined in vaccinated healthcare workers at the first, third and sixth months after the vaccination. MAGLUMI SARS-CoV-2 S-RBD IgG (CLIA) (Snibe Diagnostics, Chinese) kit was used for the detection of antibodies. The mean age of the healthcare workers was 38.93 ± 10.59 (min= 21-max= 64). After the administration of the second vaccine dose, the participants were found to be reactive for anti-SRBD-IgG antibodies by 98.2% and 97.8% at the first and third months, respectively,. 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 (first month: 42.4 AU/ml versus third month: 18.2 AU/ml). In the cohort with a history of coronavirus disease 2019 (COVID-19) prior to the vaccination, the decrease was observed as 25.1% (first month: 58.29 versus third month: 43.64 AU/ml) and at a mean of 57.4 (0-90) days prior to vaccination, the decrease rate was 43.1% (first month: 55.05 AU/ml versus third 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 first and 59.08 AU/ml at third months). At the end of the sixth month, antibodies could not be detected in 16.7% of people who (n= 42) received two doses of the inactivated vaccine, and the amount of anti-S-RDB IgG decreased by an average of 52.5% compared to the third month, and by 74.8% compared to the first month. Antibody concentrations at the first, third, and sixth months after the vaccination with two doses of the inactivated SARS-CoV-2 vaccine were found to be decreased and at the sixth month, the rate of non-reactive people was 16.7%. 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 for effective herd immunity.