Seroprevalence of SARS-CoV-2 antibodies and reduced risk of reinfection through 6 months: a Danish observational cohort study of 44 000 healthcare workers
Kasper Iversen, Jonas Henrik Kristensen, Rasmus Bo Hasselbalch, Mia Marie Pries‐Heje, Pernille Brok Nielsen, Andreas Dehlbæk Knudsen, Kamille Fogh, Jakob Norsk, Ove Andersen, Thea Kølsen Fischer, Claus Antonio Juul Jensen, Christian Torp‐Pedersen, Jørgen Rungby, Sisse B. Ditlev, Ida Hageman, Rasmus Møgelvang, Mikkel Gybel‐Brask, Ram Benny Dessau, Erik Sørensen, Lene Holm Harritshøj, Fredrik Folke, Curt Sten, Maria Elizabeth Engel Møller, Thomas Benfield, Henrik Ullum, Charlotte Sværke Jørgensen, Christian Erikstrup, Sisse Rye Ostrowski, Susanne Dam Nielsen, Henning Bundgaard
Abstract
OBJECTIVES: Antibodies to severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) are a key factor in protecting against coronavirus disease 2019 (COVID-19). We examined longitudinal changes in seroprevalence in healthcare workers (HCWs) in Copenhagen and the protective effect of antibodies against SARS-CoV-2. METHODS: In this prospective study, screening for antibodies against SARS-CoV-2 (ELISA) was offered to HCWs three times over 6 months. HCW characteristics were obtained by questionnaires. The study was registered at ClinicalTrials.gov, NCT04346186. RESULTS: From April to October 2020 we screened 44 698 HCWs, of whom 2811 were seropositive at least once. The seroprevalence increased from 4.0% (1501/37 452) to 7.4% (2022/27 457) during the period (p < 0.001) and was significantly higher than in non-HCWs. Frontline HCWs had a significantly increased risk of seropositivity compared to non-frontline HCWs, with risk ratios (RRs) at the three rounds of 1.49 (95%CI 1.34-1.65, p < 0.001), 1.52 (1.39-1.68, p < 0.001) and 1.50 (1.38-1.64, p < 0.001). The seroprevalence was 1.42- to 2.25-fold higher (p < 0.001) in HCWs from dedicated COVID-19 wards than in other frontline HCWs. Seropositive HCWs had an RR of 0.35 (0.15-0.85, p 0.012) of reinfection during the following 6 months, and 2115 out of 2248 (95%) of those who were seropositive during rounds one or two remained seropositive after 4-6 months. The 133 of 2248 participants (5.0%) who seroreverted were slightly older and reported fewer symptoms than other seropositive participants. CONCLUSIONS: HCWs remained at increased risk of infection with SARS-CoV-2 during the 6-month period. Seropositivity against SARS-CoV-2 persisted for at least 6 months in the vast majority of HCWs and was associated with a significantly lower risk of reinfection.