Effectiveness of the First and Second Severe Acute Respiratory Syndrome Coronavirus 2 Vaccine Dose: A Nationwide Cohort Study From Austria on Hybrid Versus Natural Immunity

Abstract

Objectives To evaluate the effectiveness of SARS-CoV-2 vaccinations in previously SARS-CoV-2 infected adults in the general population of Austria during the Delta wave and with extended follow-up. Methods In a nationwide retrospective cohort study, we calculated age, gender, and nursing home residency adjusted Cox proportional hazard ratios (HRs) of COVID-19 deaths, SARS-CoV-2 infections, and non-COVID-19 deaths from October 1st to December 31st, 2021, and secondarily with extended follow-up to June 30, 2022. Relative vaccine effectiveness (rVE) is rVE=(1-HR)x100. Results Among 494,646 previously infected adults, 169,543 had received two vaccine doses, 133,567 had received one dose and 190,275 were unvaccinated at baseline. We recorded 17 COVID-19 deaths (6 vaccinated, 11 unvaccinated) and 8,209 SARS-CoV-2 infections. Absolute risk of COVID-19 deaths was 0.003%. rVE estimates for COVID-19 deaths and reinfections exceeded 75% until end-2021 but decreased substantially with extended follow-up. The risk of non-COVID-19 death was lower in those vaccinated versus the unvaccinated. Discussion First and second SARS-CoV-2 vaccine doses appear effective in the short-term, but with diminishing effectiveness over time. The extremely low COVID-19 mortality, regardless of vaccination, indicates strong protection of previous infection against COVID-19 death. Lower non-COVID-19 mortality in the vaccinated population might suggest a healthy vaccinee bias.