Identification of Natural SARS-CoV-2 Infection in Seroprevalence Studies Among Vaccinated Populations

Ryan T. Demmer, Brett Baumgartner, Talia D. Wiggen, Angela K. Ulrich, Ali J. Strickland, Brianna M. Naumchik, Bruno Bohn, Sara Walsh, Stephen Smith, Susan Kline, Steve D. Stovitz, Stephanie Yendell, Timothy J. Beebe, Craig Hedberg

Research output: Contribution to journalArticlepeer-review

11 Scopus citations


Most SARS-CoV-2 antibody assays cannot distinguish between antibodies that developed after natural infection and those that developed after vaccination. We assessed the accuracy of a nucleocapsid-containing assay in identifying natural infection among vaccinated individuals. A longitudinal cohort composed of health care workers in the Minneapolis/St. Paul area was enrolled. Two rounds of seroprevalence studies separated by 1 month were conducted from November 2020 to January 2021 among 81 participants. Capillary blood from rounds 1 and 2 was tested for IgG antibodies against spike proteins by enzyme-linked immunosorbent assay (spike-only assay). During round 2, IgGs reactive to SARS-CoV-2 nucleocapsid protein (nucleocapsid-containing assay) were assessed. Vaccination status at round 2 was determined by self-report. Area under the curve was computed to determine the discriminatory ability of the nucleocapsid-containing assay for identification of recent infection. Participants had a mean age of 40 years (range, 23 to 66 years); 83% were female. Round 1 seroprevalence was 9.5%. Before round 2 testing, 46% reported vaccination. Among those not recently infected, in comparing vaccinated vs unvaccinated individuals, elevated levels of spike 1 (P<.001) and spike 2 (P=.01) were observed, whereas nucleocapsid levels were not statistically significantly different (P=.90). Among all participants, nucleocapsid response predicted recent infection with an area under the curve of 0.93 (95% CI, 0.88 to 0.99). Among individuals vaccinated more than 10 days before antibody testing, the specificity of the nucleocapsid-containing assay was 92%, whereas the specificity of the spike-only assay was 0%. An IgG assay identifying reactivity to nucleocapsid protein is an accurate predictor of natural infection among a partially vaccinated population, whereas a spike-only assay performed poorly.

Original languageEnglish (US)
Pages (from-to)754-760
Number of pages7
JournalMayo Clinic Proceedings
Issue number4
StatePublished - Apr 2022

Bibliographical note

Funding Information:
Grant Support: This study was supported by funding from the Minnesota Department of Health Contract Number 183558. Funding to establish the cohort was provided by The University of Minnesota Office of the Vice President for Research, the Minnesota Population Center (funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development Population Research Infrastructure Program P2C HD041023 ), and The University of Minnesota’s NIH Clinical and Translational Science Award UL1TR002494 . Dr Ulrich was supported by NIH grant T32AI05543315 .

Publisher Copyright:
© 2022 Mayo Foundation for Medical Education and Research


  • Adult
  • Aged
  • Antibodies, Viral
  • COVID-19/diagnosis
  • Female
  • Humans
  • Male
  • Middle Aged
  • Nucleocapsid Proteins
  • SARS-CoV-2
  • Seroepidemiologic Studies
  • Young Adult

PubMed: MeSH publication types

  • Journal Article


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