SARS-CoV-2-specific B- and T-cell immunity in a population-based study of young Swedish adults

J Allergy Clin Immunol. 2022 Jan;149(1):65-75.e8. doi: 10.1016/j.jaci.2021.10.014. Epub 2021 Oct 23.


Background: Young adults are now considered major spreaders of coronavirus disease 2019 (COVID-19) disease. Although most young individuals experience mild to moderate disease, there are concerns of long-term adverse health effects. The impact of COVID-19 disease and to which extent population-level immunity against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) exists in young adults remain unclear.

Objective: We conducted a population-based study on humoral and cellular immunity to SARS-CoV-2 and explored COVID-19 disease characteristics in young adults.

Methods: We invited participants from the Swedish BAMSE (Barn [Children], Allergy Milieu, Stockholm, Epidemiology) birth cohort (age 24-27 years) to take part in a COVID-19 follow-up. From 980 participants (October 2020 to June 2021), we here present data on SARS-CoV-2 receptor-binding domain-specific IgM, IgA, and IgG titers measured by ELISA and on symptoms and epidemiologic factors associated with seropositivity. Further, SARS-CoV-2-specific memory B- and T-cell responses were detected for a subpopulation (n = 108) by ELISpot and FluoroSpot.

Results: A total of 28.4% of subjects were seropositive, of whom 18.4% were IgM single positive. One in 7 seropositive subjects was asymptomatic. Seropositivity was associated with use of public transport, but not with sex, asthma, rhinitis, IgE sensitization, smoking, or body mass index. In a subset of representative samples, 20.7% and 35.0% had detectable SARS-CoV-2 specific B- and T-cell responses, respectively. B- and T-cell memory responses were clearly associated with seropositivity, but T-cell responses were also detected in 17.2% of seronegative subjects.

Conclusions: Assessment of IgM and T-cell responses may improve population-based estimations of SARS-CoV-2 infection. The pronounced surge of both symptomatic and asymptomatic infections among young adults indicates that the large-scale vaccination campaign should be continued.

Keywords: COVID-19 disease; IgA; IgG; IgM; SARS-CoV-2; asthma; memory B cells; memory T cells; population-based cohort; risk factors; young adults.

Publication types

  • Clinical Trial

MeSH terms

  • Adult
  • Antibodies, Viral / immunology
  • Birth Cohort
  • COVID-19 / immunology*
  • Female
  • Follow-Up Studies
  • Humans
  • Immunity, Cellular*
  • Immunity, Humoral*
  • Male
  • Memory B Cells / immunology*
  • Prospective Studies
  • SARS-CoV-2 / immunology*
  • Sweden
  • T-Lymphocytes / immunology*


  • Antibodies, Viral