Longitudinal Study of the immunological response to the SARS-CoV-2 vaccine in IPO of Coimbra professionals

• IgG levels declined over time in both seropositive and seronegative participants: • Seropositive healthcare workers showed consistently higher IgG titers after vaccination; • IgG after the 2 nd dose in seronegative were similar to the IgG after the 1 st dose in seropositive; • Participants with first-dose reactions showed higher IgG levels than those without reactions; • Our real-world data reinforce the importance of booster doses in public health planning. Objectives: Vaccination is crucial for preventing the spread of the new wave of the COVID-19 pandemic. mRNA-based vaccines were approved in Europe, Pfizer/BioNTech- BNT162b2 and Moderna- mRNA1237. This longitudinal study evaluated the humoral immune response to SARS-CoV-2 vaccination in a cohort of healthcare workers receiving two vaccine doses and a booster. Methods: Blood samples were collected before and after vaccination to understand the dynamics of IgG levels over a long period. Vaccine adverse reactions (VARs) were monitored through a questionnaire. Results: Seronegative and seropositive individuals presented a decline in IgG titters, with consistently higher titters in seropositive individuals. Vaccination may elicit a response similar in magnitude to natural infection, since the IgG titters after the second dose in seronegative individuals were similar to the IgG titters after the first dose in seropositive individuals. When sex was analysed individually, there was a greater peak in IgG titters in females. Volunteers who reported VARs, to the first dose, had higher IgG levels than did those who did not report VAR. Conclusion: Importantly, our cohort was composed mostly of females, included few seropositive individuals, and elderly people were not included. This study, which was conducted over approximately 600 days, presents real-world data on the immune response to COVID-19 vaccination from a large cohort of healthcare workers.