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BackgroundThe first Corona Monitoring Nationwide (RKI-SOEP) study (October 2020-February 2021) found a low pre-vaccine SARS-CoV-2 antibody seroprevalence (2.1%) in the German adult population (≥ 18 years).AimThe objective of this second RKI-SOEP (RKI-SOEP-2) study in November 2021-March 2022 was to estimate the prevalence of SARS-CoV-2-specific anti-spike and/or anti-nucleocapsid (anti-N) IgG antibodies (combined seroprevalence), past infection based on infection-induced seroprevalence (anti-N), and basic immunisation (at least two antigen contacts through vaccination or infection) in individuals aged ≥ 14 years. We also aimed to estimate under-reporting of infections.MethodsDried blood-spot specimens from a population-based sample embedded in a dynamic cohort, the Socio-Economic Panel (SOEP), were serologically analysed. Resulting serological data and self-reports via a questionnaire from the same individuals were used to estimate prevalences.ResultsCombined seroprevalence was 90.7% (95% CI: 89.7%-91.6%) without correction and 94.6% (95% CI: 93.6%-95.7%) with correction for sensitivity/specificity and antibody waning. While one in nine individuals had been infected (11.3%; 95% CI: 9.1%-13.5%), nine in 10 had a basic immunisation (90%; 95% CI: 88.9-90.9%), primarily due to vaccination. Population-weighted estimates differed by age, region, and socioeconomic deprivation. The under-reporting factor was estimated as 1.55 (95% CI: 1.3-1.8).ConclusionsWhen the SARS-CoV-2-Omicron wave was beginning, most people had been vaccinated, infected, or both. Large-scale vaccination, but not a high infection rate, was able to fill the immunity gap, especially in ≥ 65 year-olds who are known to be at higher risk of severe COVID-19. Our data point towards the need for targeted socioeconomically, demographically and regionally stratified mitigation strategies, including measures to enhance vaccine uptake.