Population-based seroprevalence of prior infection with COVID-19 in Canada: implications for testing, economic revitalization and population health.

Canadian Blood Services blood donors are healthy people from cities and smaller urban areas in all provinces in Canada except Quebec. Left over blood samples from blood donations will be randomly selected and tested for antibodies to COVID-19. Antibodies are part of the body's response to an infection and are specific for the infecting virus. The presence of antibodies to COVID-19 means that the person previously had COVID-19. They may or may not have immunity to prevent them from getting COVID-19 again (that is, have functional immunity). Over 12 months, 1,500 samples will be tested each month. All samples that test positive for COVID-19 antibodies will have additional testing to measure the antibody concentration (titer) and how effective the antibodies are at binding to COVID-19 virus (neutralization). These will help to understand how likely people who had COVID-19 are to have immunity against re-infection. Tests that can be used for large numbers of samples will be compared to help Canada develop tests suitable for large scale testing. Monitoring functional immunity over the course of the pandemic will help to understand if people start to lose their immunity over time. Monthly data reports will be made available to public health staff (federal and provincial) to inform policy over the course of the pandemic. Mathematical modelling will refine estimates of immunity in the population which is fundamental to public health policy formulation.