Seroprevalence and levels of IgG antibodies after COVID-19 infection or vaccination

Infect Dis (Lond). 2022 Jan;54(1):63-71. doi: 10.1080/23744235.2021.1974540. Epub 2021 Sep 14.


Background: In a country-wide seroprevalence study of COVID-19 in Estonia, we aimed to determine the seroprevalence and the dynamics of IgG against SARS-CoV-2 after vaccination or positive PCR-test.

Methods: Leftover blood samples were selected between 8 February and 25 March 2021, by SYNLAB Estonia from all counties and age groups (0-9, 10-19, 20-59, 60-69, 70-79 and 80-100 years) proportionally to the whole Estonian population and tested for IgG against SARS-CoV-2 spike protein receptor-binding domain (anti-S-RBD IgG) using Abbott SARS-CoV-2 IgG II Quant assay. Antibody levels after positive PCR-test or vaccination were described by exponential increase-decrease models.

Results: According to total of 2517 samples, overall seroprevalence (95% confidence interval [CI]) was 20.1% (18.5-21.7%), similar in all age groups, but varied between counties. If individuals vaccinated with the first dose at least 14 d before antibody measurement were assumed to be seronegative, the overall seroprevalence was 15.8% (14.4-17.3%), 4.0-fold larger than the proportion of PCR-confirmed COVID-19 cases. Of seropositive individuals (n = 506) 194 (38.3%; 33.8-43.1%) had not had positive PCR-test or been vaccinated. According to exponential increase-decrease model, the peak of anti-S-RBD IgG in a 52-year-old (median age of PCR-positive and/or vaccinated individuals) was significantly higher after vaccination compared with positive PCR-test (22,082 (12,897-26,875) vs. 6732 (2321-8243) AU/mL), but half-life was similar (26.5 (6.9-46.1) vs. 38.3 (8.2-68.5) d).

Conclusions: One year after the start of COVID-19 pandemic the actual prevalence of infection is still underestimated compared with PCR-confirmed COVID-19 cases. Older compared with younger individuals have lower anti-S-RBD IgG level after vaccination, but similar decline rate.

Keywords: Antibodies; Cross-sectional study; Estonia; SARS-CoV-2 PCR; mRNA-vaccine.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Adult
  • Aged
  • Aged, 80 and over
  • Antibodies, Viral / blood*
  • COVID-19 Vaccines / immunology*
  • COVID-19* / immunology
  • COVID-19* / prevention & control
  • Child
  • Child, Preschool
  • Estonia
  • Humans
  • Immunoglobulin G / blood*
  • Infant
  • Infant, Newborn
  • Middle Aged
  • Pandemics
  • Seroepidemiologic Studies
  • Spike Glycoprotein, Coronavirus / immunology
  • Vaccination
  • Young Adult


  • Antibodies, Viral
  • COVID-19 Vaccines
  • Immunoglobulin G
  • Spike Glycoprotein, Coronavirus
  • spike protein, SARS-CoV-2

Grants and funding

The study was funded by the Ministry of Social Affairs of the Republic of Estonia.