Abstract
As national coronavirus disease 2019 (COVID-19) mass vaccination campaigns are rolled out, monitoring real-world Vaccine Effectiveness (VE) and its durability is essential. We aimed to estimate COVID-19 VE against severe disease and death in the Greek population, for all vaccines currently in use. Nationwide active surveillance and vaccination registry data during January-December 2021 were used to estimate VE via quasi-Poisson regression, adjusted for age and calendar time. Interaction terms were included to assess VE by age group, against the "delta" severe acute respiratory syndrome coronavirus 2 variant and waning of VE over time. 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. VE waned over time but remained >80% at 6 months, and three doses increased VE again to near 100%. Vaccination prevented an estimated 19 691 COVID-19 deaths (95% confidence interval: 18 890-20 788) over the study period. All approved vaccines offer strong and also durable protection against COVID-19 severe disease and death. Every effort should be made to vaccinate the population with at least two doses, to reduce the mortality and morbidity impact of the pandemic.