High prevalence of undocumented SARS-CoV-2 infections revealed by analysis of nucleocapsid-specific IgG responses in diagnosed and undiagnosed individuals.

Acute SARS-CoV-2 infections are not always diagnosed; hence an unknown proportion of all infections are not documented. SARS-CoV-2 can induce spike and nucleocapsid protein specific IgG antibodies, which can be detected in seroprevalence studies to identify a previous infection. However, with the introduction of vaccines containing the spike protein it is no longer possible to use spike-IgG as a marker of infection.

High Prevalence of SARS-CoV-2 Omicron Infection Despite High Seroprevalence, Sweden, 2022.

We performed 2 surveys during 2022 to estimate point prevalences of SARS-CoV-2 infection compared with overall seroprevalence in Sweden. Point prevalence was 1.4% in March and 1.