Effectiveness of COVID-19 Vaccines Against Severe Disease and Death and its Durability Over Time Against the Omicron Variant

ABSTRACT

Background. We aimed to estimate COVID-19 Vaccine Effectiveness (VE) and its durability against severe disease (intubation) and death in the Greek population, for all vaccines currently in use. Methods. Nationwide active surveillance and vaccination registry data between January 2021 - April 2022 were used to estimate VE via quasi-Poisson regression as one minus Incidence Rate Ratio, adjusted for age and calendar time. Interaction terms were included to assess VE by age group, by variant (delta vs omicron) and by time since vaccination (compared to the first month after vaccination). Results. VE results are summarized in Figures 1-3. After 2-doses (of any vaccine) VE was >95% against delta and >85% against omicron in the 15-59 and 60-79 age groups, but waned significantly at 6 months by 5-10% in the 15-59 group and 15-20% in the 60-79 group. In the 80+ age group, 2-dose VE was much lower against omicron at around 60%, waning to 50% at 6 months. With 3 vaccine doses VE surged to >95% against both delta and omicron in all age groups except 80+, who had VE against omicron of =90%;however there was again substantial waning at 6 months by about 10-15% in all groups. In the 80+ age group, VE against omicron dropped to 81.4% (95% CI 76.7-85.2%) for death and 57.7% (95% CI 38.5-70.9%) for intubation. There was little to no evidence of variation in VE between combinations of different vaccines (BNT162b2, mRNA-1273, ChAdOx1 nCov-19 and Ad26.COV2.S), although the majority of the population received 2- or 3-dose BNT162b2. Vaccination prevented an estimated 39,018 COVID-19 deaths (95% CI 37,791- 40,608) over the study period. Conclusion. COVID-19 vaccination remains extremely effective in preventing severe disease and death, even in the context of the omicron variant, as long as 3 doses have been administered. Even after 3 doses, however, effectiveness wanes substantially in the course of 6 months. Additional strategies will be therefore needed in order to minimize COVID-19 mortality and morbidity, including boosting doses especially in the elderly, improved vaccines, early diagnosis and treatment of high-risk groups and measures to reduce community transmission of current or novel SARS-CoV-2 immune escape variants.