Vaccine effectiveness of primary and booster COVID-19 vaccinations against SARS-CoV-2 infection in the Netherlands from 12 July 2021 to 6 June 2022: a prospective cohort study (preprint)

ABSTRACT

Introduction - Monitoring of COVID-19 vaccine effectiveness (VE) is needed to inform vaccine policy. We estimated VE of primary vaccination, and first and second booster vaccination, against SARS-CoV-2 infection overall, and in four risk groups defined by age and medical risk condition, in the Delta and Omicron BA.1/BA.2 periods. Methods - VASCO is an ongoing prospective cohort study among vaccinated and unvaccinated Dutch adults. The primary endpoint was a self-reported positive SARS-CoV-2 test during 12 July 2021-6 June 2022. Participants with a prior SARS-CoV-2 infection, based on a positive test or serology, were excluded. We used Cox proportional hazard models with vaccination status as time-varying exposure and adjustment for age, sex, educational level, and medical risk condition. We stratified by Delta and Omicron BA.1/BA.2 periods, risk group, and time since vaccination. Results - 37,170 participants (mean age 57 years) were included. In the Delta period, VE <6 weeks after primary vaccination was 80% (95%CI 69-87) and decreased to 71% (65-77) after 6 months. VE increased to 96% (86-99) shortly after the first booster vaccination. In the Omicron period these estimates were 46% (22-63), 25% (8-39) and 57% (52-62), respectively. VE was 50% (34-62) <6 weeks after a second booster vaccination in participants aged [≥]60 years. For the Omicron period, an interaction term between vaccination status and risk group significantly improved the model (p<0.001), with generally lower VEs for those with a medical risk condition. Conclusions - Our results show the benefit of booster vaccinations against infection, also in risk groups, although the additional protection wanes quite rapidly.