Comparative effectiveness of COVID-19 vaccination against death and severe disease in an ongoing nationwide mass vaccination campaign

Background As national COVID-19 mass vaccination campaigns are rolled out, it is important to demonstrate and measure their public health benefit. We aimed to estimate COVID-19 Vaccine Effectiveness (VE) against severe disease and death in the Greek population, for all vaccines in use. Methods Nationwide active surveillance and vaccination registry data during January-December 2021 were used to estimate VE via quasi-Poisson regression, as one minus the Incidence Rate Ratio, adjusted for age and calendar time. Interaction terms were included to assess VE by age group, against the “delta” SARS-CoV-2 variant and waning of VE over time. Results Two doses of BNT162b2, mRNA-1273 or ChAdOx1 nCov-19 vaccines offered very high (>90%) VE against both intubation and death across all age groups, similar against both “delta” and previous variants, with one-dose Ad26.COV2.S slightly lower. There was some waning over time but VE remained >80% at six months, and three doses increased VE again to near 100%. Vaccination prevented an estimated 19,691 COVID-19 deaths (95% CI: 18,890-20,788) over the study period. Conclusions All approved vaccines were very highly effective in preventing COVID-19 severe disease and death. Every effort should be made to vaccinate the population with at least two doses, in order to reduce the mortality and morbidity impact of the pandemic.