Kinetics and seroprevalence of anti-SARS-CoV-2 antibodies in vaccinated and unvaccinated health workers in Bamako, Mali, November 2021-February 2023.

Background: This prospective study aimed to evaluate the dynamics and seroprevalence of anti-SARS-CoV-2 antibodies in a cohort of vaccinated and unvaccinated health workers (HWs) in Bamako, Mali. The study also measured antibody responses as a function of SARS-CoV-2 infections, socio-demography, vaccination status and associated comorbidities.

Method: 685 vaccinated and 413 unvaccinated HWs (total = 1098) were monitored over a 15-month periods with follow-up visits every 3 months for the first 6 months and a final visit after 15 months. Anti-spike and anti-nucleoprotein antibodies were assessed using ELISA. Non-parametric tests and logistic regression analyses were performed to compare antibody levels and to investigate associated factors.

Results: Overall, anti-SARS-CoV-2 antibodies tended to increase over the first 3 months in both vaccinated and unvaccinated groups. Spike (>98 %) and nucleoprotein (>91 %) antibodies remained high and relatively stable after 3 months and correlated inversely with the rate attack of symptomatic and asymptomatic COVID-19. Males demonstrated lower antibody responses, particularly for nucleoprotein (p < 0.05) compared to females. HWs with comorbidities demonstrated higher antibody responses. Participants with active SARS-CoV-2 infections exhibited decreased antibody levels. Vaccinated participants exhibited a trend toward higher spike antibodies. Virus-inactivated type vaccination increased nucleoprotein antibodies. Furthermore, there was no increase of antibody levels after receiving, two or more vaccine doses, independently of whether the same or a different type of used vaccines. Nucleoprotein antibodies varied significantly at inclusion among participants across blood groups.

Conclusion: This study identified a progressive increase in anti-spike and anti-nucleoprotein antibodies in both vaccinated and unvaccinated HWs, correlating with reduced COVID-19 infection rates. These findings highlight the significance of both natural and vaccine-induced immunity and the need to assess antibody levels and long-term protection beyond the 15-month study period.