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

Björkander, Sophia; Du, Likun; Zuo, Fanglei; Ekström, Sandra; Wang, Yating; Wan, Hui; Sherina, Natalia; Schoutens, Lisanne; Andréll, Juni; Andersson, Niklas; Georgelis, Antonios; Bergström, Anna; Marcotte, Harold; Kull, Inger; Hammarström, Lennart; Melén, Erik; Pan-Hammarström, Qiang

Background Young adults are now considered major spreaders of COVID-19 disease. Although most young individuals suffer from 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 SARS-CoV-2 exist in young adults remain unclear. Objective To conduct a population-based study on humoral and cellular immunity to SARS-CoV-2 and explore COVID-19 disease characteristics in young adults. Methods We invited participants from the Swedish BAMSE birth cohort (age 24-27 years) to take part in a COVID-19 follow-up. From 980 participants (October 2020-June 2021), we here present data on SARS-CoV-2 RBD-specific IgM, IgA and IgG titres measured by ELISA and on symptoms and epidemiological 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 28.4% of subjects were seropositive of which 18.4% were IgM single positive. One in seven seropositive subjects were asymptomatic. Seropositivity associated with use of public transport, but not with sex, asthma, rhinitis, IgE-sensitization, smoking or BMI. 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. Conclusion 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 should imply a continuation of the large-scale vaccination campaign.