Assessment of population infection with SARS-CoV-2 in Ontario, Canada, March to June 2020

Background: Serosurveys for SARS-CoV-2 aim to estimate the proportion of the population that has been infected. Aim: This observational study assesses the seroprevalence of SARS-CoV-2 antibodies in Ontario, Canada during the first pandemic wave. Methods: Using an orthogonal approach, we tested 8,902 residual specimens from the Public Health Ontario laboratory over three time periods during March-June 2020 and stratified results by age group, sex and region. We adjusted for antibody test sensitivity/specificity and compared with reported PCR-confirmed COVID-19 cases. Results: Adjusted seroprevalence was 0.5% (95% confidence interval (CI): 0.1-1.5) from 27 March-30 April, 1.5% ( 95% CI: 0.7-2.2) from 26-31 May, and 1.1% (95% CI: 0.8-1.3) from 5-30 June 2020. Adjusted estimates were highest in individuals aged >= 60 years in March-April (1.3%; 95% CI: 0.2-4.6), in those aged 20-59 years in May (2.1%; 95% CI: 0.8-3.4) and in those aged >= 60 years in June (1.6%; 95% CI: 1.1-2.1). Regional seroprevalence varied, and was highest for Toronto in March-April (0.9%; 95% CI: 0.1-3.1), for Toronto in May ( 3.2%; 95% CI: 1.0-5.3) and for Toronto (1.5%; 95% CI: 0.9-2.1) and Central East in June (1.5%; 95% CI: 1.0-2.0). We estimate that COVID-19 cases detected by PCR in Ontario underestimated SARS-CoV-2 infections by a factor of 4.9. Conclusions: Our results indicate low population seroprevalence in Ontario, suggesting that public health measures were effective at limiting the spread of SARS-CoV-2 during the first pandemic wave.