Longitudinal SARS‐CoV‐2 seroprevalence in Portugal and antibody maintenance 12 months after infection

During the COVID‐19 pandemic, Portugal has experienced three distinct SARS‐CoV‐2 infection waves. We previously documented the prevalence of SARS‐CoV‐2 immunity, measured by specific antibodies, in September 2020, 6 months after the initial moderate wave. Here, we show the seroprevalence changes 6 months later, up to the second week of March 2021, shortly following the third wave, which was one of the most severe in the world, and 2 months following the start of the vaccination campaign. A longitudinal epidemiological study was conducted, with a stratified quota sample of the Portuguese population. Serological testing was performed, including ELISA determination of antibody class and titers. The proportion of seropositives, which was 2.2% in September 2020, rose sharply to 17.3% (95% CI: 15.8–18.8%) in March 2021. Importantly, circulating IgG and IgA antibody levels were very stable 6 months after the initial determination and up to a year after initial infection, indicating long‐lasting infection immunity against SARS‐CoV‐2. Moreover, vaccinated people had higher IgG levels from 3 weeks post‐vaccination when compared with previously infected people at the same time post‐infection.