Seropositivity in blood donors and pregnant women during 9-months of SARS-CoV-2 transmission in Stockholm, Sweden

Public health strategies to contain the pandemic continue to vary markedly across the world. In Sweden, compared to most advanced economies, social restrictions have primarily relied upon voluntary adherence to a set of recommendations and strict lockdowns have not been enforced. To better understand the development of humoral immunity to SARS-CoV-2 in the Stockholm population before the start of mass vaccinations, healthy blood donors and pregnant women (n=4,100) were sampled at random between 14th March-11th December 2020. All individuals (n=200/sampling week) were screened for anti-SARS-CoV-2 spike (S) trimer- and RBD-specific IgG responses with highly sensitive and specific ELISA assays, and the results were compared with those from historical controls (n=595). Data were modelled using a probabilistic Bayesian framework that considered individual responses to both antigens. We found that after a steep rise at the start of the pandemic, the seroprevalence trajectory increased steadily in approach to the winter second-wave of infections, approaching 15% of all individuals surveyed by 11th December. In agreement with the high transmission rate observed in the Stockholm area, seroprevalence in this cohort of active adults increased during the 9 months from the start of the outbreak, but was far from that required for herd immunity at the end of 2020.