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

Abstract

Studying the prevalence of SARS-CoV-2 specific antibodies (seroprevalence) allows assessing the impact of epidemic containment measures and vaccinations, as well as estimation of 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 youngest age groups from Q2/2022 onwards. 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.

Hybrid immunity improves the immune response after fourth Covid-19 vaccine dose in individuals with medical conditions predisposing to severe Covid-19

“…The exposure of different population groups to infections may have varied, especially during the first epidemic waves, when containment measures were widely used. The proportion of immunocompromised participants who became infected (31%) during the first Omicron wave during this study, was, however, similar to the age-standardized seroprevalence of the population (39 %) in the Finnish population study carried out at the same time [16]. Considering that we excluded those participants who were infected before receiving the fourth vaccine dose, the cumulative seroprevalence in the immunocompromised participants would probably have been even closer to the general population.…”

“…With the spread of Omicron variants, vaccinations have provided only very limited and short-lived protection from Covid-19 infections even in the boosted immunocompetent population. We have estimated, that 39% of the Finnish adult population had developed antibody-mediated hybrid immunity induced by a combination of vaccinations and infections during the first half of 2022, and over half by the end of 2022 [16]. Data on development of the hybrid immunity in different immunocompromised disease groups is scarce.…”

High‐Dose Quadrivalent Influenza Vaccine for Prevention of Cardiovascular and Respiratory Hospitalizations in Older Adults

“…Indeed, rVE estimates were slightly higher after excluding COVID‐19‐linked secondary discharge codes (Figure 3). Additional hospitalizations with undetected COVID‐19 may have occurred early in the pandemic, when there were limited diagnostic resources, although SARS‐CoV‐2 seroprevalence remained low [27]. In mid‐March 2020, Finland introduced extensive social restrictions, after which influenza circulation declined rapidly (Figure S6) [25, 28].…”