The evolution of SARS-CoV-2 seroprevalence in Canada: a time-series study, 2020-2023.

Background: During the first year of the COVID-19 pandemic, the proportion of reported cases of COVID-19 among Canadians was under 6%. Although high vaccine coverage was achieved in Canada by fall 2021, the Omicron variant caused unprecedented numbers of infections, overwhelming testing capacity and making it difficult to quantify the trajectory of population immunity.
Methods: Using a time-series approach and data from more than 900 000 samples collected by 7 research studies collaborating with the COVID-19 Immunity Task Force (CITF), we estimated trends in SARS-CoV-2 seroprevalence owing to infection and vaccination for the Canadian population over 3 intervals: prevaccination (March to November 2020), vaccine roll-out (December 2020 to November 2021), and the arrival of the Omicron variant (December 2021 to March 2023). We also estimated seroprevalence by geographical region and age.
Results: By November 2021, 9.0% (95% credible interval [CrI] 7.3%-11%) of people in Canada had humoral immunity to SARS-CoV-2 from an infection. Seroprevalence increased rapidly after the arrival of the Omicron variant - by Mar. 15, 2023, 76% (95% CrI 74%-79%) of the population had detectable antibodies from infections. The rapid rise in infection-induced antibodies occurred across Canada and was most pronounced in younger age groups and in the Western provinces: Manitoba, Saskatchewan, Alberta and British Columbia.
Interpretation: Data up to March 2023 indicate that most people in Canada had acquired antibodies against SARS-CoV-2 through natural infection and vaccination. However, given variations in population seropositivity by age and geography, the potential for waning antibody levels, and new variants that may escape immunity, public health policy and clinical decisions should be tailored to local patterns of population immunity.
Competing Interests: Competing interests: Maureen Anderson reports receiving a Saskatchewan Health Research Foundation Establishment Grant, in support of the present manuscript. Lesley Behl reports receving funding from the CITF via a research grant paid to the University of Saskatchewan, in support of the present manuscript. Patrick Brown reports receiving grant support from the CITF. Carmen Charlton reports receiving grant funding from Merck and support for attending meetings or travel from AMMI Canada. Karen Colwill reports receiving an honorarium for a presentation to the Ontario Society of Clinical Chemists. Dr. Colwill is also the vice president of communications for the Canadian National Proteomics Network. Tim Evans reports receiving support for the present manuscript from the CITF, and holds the role of its executive director. Anne-Claude Gingras reports receiving support for automated equipment for ELISA, housed at the Lunenfeld-Tanenbaum Research Institute (a facility supported by Canada Foundation for Innovation funding, the Ontario Government, Genome Canada and Ontario Genomics); support from the Canadian Institutes of Health Research (CIHR) for general lab operations; salary support as a Canada Research Chair (Tier 1) in functional proteomics; and from CITF and the Public Health Agency of Canada (PHAC) for assay calibration at Mount Sinai, all in support of the present manuscript. Dr. Gingras has had a contract with Providence Therapeutics (no consultation fees received), and has received grant funding from CIHR for several SARS-CoV-2 studies and from the Ontario Research Fund, outside of the present work. Dr. Gingras has served as the chair of the Institute Advisory Board of the CIHR Institute of Genetics and is a member and chair of the Human Health Therapeutics Division advisory board, National Research Council of Canada. Dr. Gingras is a member of the CITF working parties on testing and immunology, and is pillar lead of the Functional Genomics and Structure Function of CoVaRR-Net (all roles unpaid). Steven Drews reports receiving grants or contracts from CIHR and Alberta Innovates, consulting fees from Roche, support for attending meetings or travel from the Canadian Immunization Research Network and other support from Abbott. Jaspreet Jain reports receiving support from the CITF for the present manuscript. Prabhat Jha reports receiving grant support from the CITF for the Action to Beat Coronavirus study. Amanda Lang reports receiving funding from the CITF, in support of the present manuscript, for assay reagents and technologist salary. Dr. Lang has also received support from the CITF to attend a CITF meeting. Stephen Lee reports receiving funding from the CITF, in support of the present manuscript. Sheila O’Brien reports receiving payment (paid to Canadian Blood Services [CBS]) from the Government of Canada, and laboratory infrastructure from CBS, in support of the present manuscript. Chantale Pambrun reports receiving salary support from CBS, and study funding from PHAC. Olivia Oxlade is an employee of the CITF Secretariat. David Stephens reports receiving funds for a graduate student from the National Science and Engineering Research Council Emerging Infectious Disease Modelling, in support of the present manuscript. Hanna Swail reports receiving support from the CITF for the present manuscript. David Buckeridge reports receiving funding from PHAC, in support of the present manuscript. No other competing interests were declared.
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