{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,1,8]],"date-time":"2026-01-08T20:19:11Z","timestamp":1767903551953,"version":"3.49.0"},"reference-count":26,"publisher":"Public Library of Science (PLoS)","issue":"1","license":[{"start":{"date-parts":[[2025,1,22]],"date-time":"2025-01-22T00:00:00Z","timestamp":1737504000000},"content-version":"vor","delay-in-days":0,"URL":"http:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100004359","name":"Vetenskapsr\u00e5det","doi-asserted-by":"publisher","award":["2021-05045"],"award-info":[{"award-number":["2021-05045"]}],"id":[{"id":"10.13039\/501100004359","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100006129","name":"Konung Gustaf V:s och Drottning Victorias Frimurarestiftelse","doi-asserted-by":"publisher","id":[{"id":"10.13039\/501100006129","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":["www.plosglobalpublichealth.org"],"crossmark-restriction":false},"short-container-title":["PLOS Glob Public Health"],"abstract":"<jats:p>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. In many countries marketed vaccines do not include the nucleocapsid protein, allowing the use of nucleocapsid-specific IgG (N-IgG) as a specific marker for previous infection. Importantly however, not all SARS-CoV-2-infected individuals develop detectable N-IgG responses and there are reports of waning of N-IgG titers in previously infected individuals, complicating the use of N-IgG in seroprevalence studies. Here, our aim was to investigate N-IgG as a marker for previous infection. To this end we analyzed a well characterized cohort (n = 2,583; sampled in March, 2022), including 612 participants with a previously diagnosed and documented SARS-CoV-2-infection. We show that 75% (460\/612) of the confirmed SARS-CoV-2-infected participants were N-IgG positive, and that the frequency of seropositivity was stable for at least 105 weeks after the latest documented SARS-CoV-2-infection. Among participants with no documented SARS-CoV-2-infection, 32.6% (642\/1971) were N-IgG-positive, suggesting a previous infection. Assuming similar frequency of N-IgG-seronegative cases in previously diagnosed and undiagnosed individuals we further estimate that 214 of the 1329 undiagnosed and N-IgG-negative cases had been previously infected, indicating a total infection rate of 56.8% (1,468\/2,583), clearly higher than the documented 23.7% rate of infection, in this cohort. In conclusion, our results suggest that while N-IgG is a good marker of previous SARS-CoV-2-infection the large proportion of previously infected N-IgG-negative individuals introduces a risk for underestimations of total level of previously infected individuals in a population. Accounting for this dark number of undiagnosable cases can provide better estimates of total level of infected individuals in a population.<\/jats:p>","DOI":"10.1371\/journal.pgph.0003300","type":"journal-article","created":{"date-parts":[[2025,1,22]],"date-time":"2025-01-22T18:40:00Z","timestamp":1737571200000},"page":"e0003300","update-policy":"https:\/\/doi.org\/10.1371\/journal.pgph.corrections_policy","source":"Crossref","is-referenced-by-count":2,"title":["High prevalence of undocumented SARS-CoV-2 infections revealed by analysis of nucleocapsid-specific IgG responses in diagnosed and undiagnosed 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