This is the first population-level study of COVID19 vaccine effectiveness (VE) in Austria. We present age-group-specific estimates of effectiveness in preventing SARS-CoV-2 infection and COVID19 disease (i.e. symptomatic SARS-CoV2 infection) for the mRNA vaccines (BNT162b2 vaccine [Comirnaty from BioNTech/Pfizer]; mRNA-1273 vaccine [Spikevax from Moderna]), for the AZD1222 vaccine (Vaxzervia from AstraZeneca), and for the Ad26.COV2-S vaccine (from Janssen (Johnson & Johnson)).

Efficacy was also evaluated against the two predominant viral variants, alpha variant and delta variant, by limiting the initial study period, from week 05 to week 35, to those two time periods in which each of these variants accounted for more than 90% of the typed isolates: this results in an alpha study period from week 16-22 and a delta study period from week 27-35.

The study design is a population-based retrospective cohort study. The underlying data are national surveillance data on SARS-CoV-2 cases (source: EMS) and vaccine data from the national vaccine registry. The study used 237,702 laboratory-confirmed cases.

"Fully vaccinated" status in terms of complete vaccine protection was assumed if at least 2 weeks had elapsed after completed vaccination series (2 doses for the mRNA vaccines and AZD1222 vaccine, 1 dose for the J&J vaccine) for a 2-dose regimen and 4 weeks for the 1-dose regimen. We calculated the ratio (ratio, R) of incidence rates (incidence rate ratio, IRR) per 100 000 person-days (i.e., observation days) for the fully vaccinated group compared with the unvaccinated group using negative binomial regression models. From this, we calculated vaccine effectiveness in terms of three events, total infection (including asymptomatic and symptomatic infection), symptomatic infection, and asymptomatic infection, according to the formula VE = (1 - IRR) × 100. Including the characteristics of age (18-29, 30-39, 40-49, 50-59, 60-75, 75+ years), sex, state of residence, and calendar week, we calculated adjusted vaccine effectiveness; this is also presented separately by age group (18-39, 40-59, and ≥ 60 years) and vaccine (mRNA vaccines, AZD1222- , J&J vaccine).