Monitoring humoral immune response following COVID-19 immunization and booster dose: A prospective study in tunisian healthcare workers

As SARS-CoV-2 vaccines are deployed worldwide, assessing the kinetics and magnitude of anti-SARS-CoV-2 antibodies post-vaccination at various time points is crucial to optimize immunization strategies. This study aims to evaluate the humoral response in healthcare workers (HCWs) after primary vaccination and booster doses both in the short and long term and to examine the effect of preexisting immunity, age, sex, and vaccine type on this response. Prior to the primary vaccination, an initial serology was performed to determine the immunity to SARS-CoV-2. Based on the outcomes of this serology or the rapid diagnostic tests, participants were split into two groups: COVID-19-free and COVID-19-recovered people. In each group, serological tests were conducted one and six months following the first vaccination(M1,M6).The vaccines administrated were mRNA, viral vector and inactivated viral vaccines. The follow-up was done one and six months after the booster dose (mRNA vaccine) (M1B,M6B). Anti-SARS-CoV-2 anti-S IgG were evaluated using the Access SARS-CoV-2 IgGII® test (BECKMAN COULTER). A total of 319 HCWs were sampled. For COVID-19-recovered people, the median anti-S level was significantly higher at 130AU/mL(IQR 51.5–184) compared to 89AU/mL(IQR 34.1–145.4), p = 0.0042. At every stage, there was a statistically significant increase in anti-S levels in adenovirus or inactivated vaccinations. Participants who received mRNA vaccines had significantly the highest anti-S levels at M1, M6, and M6B, according to post-hoc analysis. The anti-S level increased significantly one month after the third dose, from 12.7AU/mL(IQR 6.4–28.4) to 140.6AU/mL(IQR 88.6–258.5), p < 0.001 . Six months after the booster dose, the anti-S titer dropped but remained positive at 40.8AU/mL(IQR22.5–91.3). In every time period, there was no correlation or association between anti-S level and age or sex. The administration of mRNA vaccines allows to an enhanced humoral response in individuals recovering from COVID-19. Six months following the initial immunization, one dose has the same immunogenicity as two doses. However, a third dose of the mRNA vaccine should be given to lengthen the duration of the humoral in both COVID-19-free and COVID-19-recovered people. Booster doses should be administrated to high risk group who can transmit the disease to susceptible patient.