Changes in SARS-CoV-2 seroprevalence and population immunity in Finland, 2020-2022

Studying the prevalence of SARS-CoV-2 specific antibodies (seroprevalence) allows for assessing the impact of epidemic containment measures and vaccinations and estimating the number of infections regardless of viral testing. We assessed antibody-mediated immunity to SARS-CoV-2 induced by infections and vaccinations from April 2020 to December 2022 in Finland by measuring serum IgG to SARS-CoV-2 nucleoprotein (N-IgG) and spike glycoprotein from randomly selected 18-85-year-old subjects (n = 9794). N-IgG seroprevalence remained at <7% until the last quartile (Q) of 2021. After the emergence of the Omicron variant, N-IgG seroprevalence increased rapidly and was 31% in Q1/2022 and 54% in Q4/2022. Seroprevalence was highest in the youngest age groups from Q2/2022 onwards. We did not observe regional differences in seroprevalence in 2022. We estimated that 51% of the Finnish 18-85-year-old population had antibody-mediated hybrid immunity induced by a combination of vaccinations and infections by the end of 2022. In conclusion, major shifts in the COVID-19 pandemic and resulting population immunity could be observed by serological testing.

Automated summary

Studying the prevalence of SARS-CoV-2 specific antibodies allows for assessing the impact of containment measures and vaccinations. In Finland, N-IgG seroprevalence remained low until 2021 and increased rapidly after the emergence of the Omicron variant. Seroprevalence was highest in the youngest age groups from 2022 onwards. By the end of 2022, it was estimated that 51% of the Finnish population had antibody-mediated hybrid immunity.