Effectiveness of a third dose of COVID-19 vaccines against delta variant of SARS-CoV-2-a Serbian cohort study

Introduction/Objective The duration of vaccine-induced protection against SARS-CoV-2 is shown to be limited. The aim of this study was to assess vaccine effectiveness (VE) of a third dose of four different COVID-19 vaccines during Delta variant predominance in Serbia. Methods The data for the period from August 18, to October 1, 2021 were used to estimate the incidence rates ( IR) of the SARS- CoV-2 infection, COVID- 19-related hospitalization, and intensive care unit (ICU) admission. The study included 41,186 fully vaccinated subjects, of which 13,589 had received the third dose. VE was estimated based on the IR ratio following vaccination with three vs. two doses. Results We found that a third dose of all investigated vaccines reduces the incidence of both SARS-CoV-2 infection and severe illness that requires hospitalization or ICU admission. The highest VE against infection demonstrated BNT162b2, followed by Gam-COVID-Vac and BBIBP-CorV. Third dose vaccination reduced the risk of hospitalization (IR = 0 for Gam-COVID-Vac and BBIBP-CorV), and ICU admission (IR = 0 for all vaccines). The hazard distributions for SARS- CoV-2 infection and hospitalization following vaccination with three versus two doses were significantly different. Conclusion These findings indicate that an additional, third dose of studied vaccine boosters protection against all investigated outcomes.