COVID Seroprevalence, Symptoms and Mortality During the First Wave of SARS-CoV-2 in Canada

  1. Action to beat coronavirus/Action pour battre le coronavirus (Ab-C) Study Investigators
  2. Prabhat Jha

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Abstract

Background

Efforts to stem Canada’s SARS-CoV-2 pandemic can benefit from direct understanding of the prevalence, infection fatality rates (IFRs), and information on asymptomatic infection.

Methods

We surveyed a representative sample of 19,994 adult Canadians about COVID symptoms and analyzed IgG antibodies against SARS-CoV-2 from self-collected dried blood spots (DBS) in 8,967 adults. A sensitive and specific chemiluminescence ELISA detected IgG to the spike trimer. We compared seroprevalence to deaths to establish IFRs and used mortality data to estimate infection levels in nursing home residents.

Results

The best estimate (high specificity) of adult seroprevalence nationally is 1.7%, but as high as 3.5% (high sensitivity) depending on assay cut-offs. The highest prevalence was in Ontario (2.4-3.9%) and in younger adults aged 18-39 years (2.5-4.4%). Based on mortality, we estimated 13-17% of nursing home residents became infected. The first viral wave infected 0.54-1.08 million adult Canadians, half of whom were <40 years old. The IFR outside nursing homes was 0.20-0.40%, but the COVID mortality rate in nursing home residents was >70 times higher than that in comparably-aged adults living in the community. Seropositivity correlated with COVID symptoms, particularly during March. Asymptomatic adults constituted about a quarter of definite seropositives, with a greater proportion in the elderly.

Interpretation

Canada had relatively low infection prevalence and low IFRs in the community, but not in nursing homes, during the first viral wave. Self-collected DBS for antibody testing is a practicable strategy to monitor the ongoing second viral wave and, eventually, vaccine-induced immunity among Canadian adults.

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  1. ScreenIT

    SciScore for 10.1101/2021.03.04.21252540: (What is this?)

    Please note, not all rigor criteria are appropriate for all manuscripts.

    Table 1: Rigor

    Institutional Review Board StatementIRB: 12 We drew on estimated IFRs from a demographic analysis of deaths in nursing homes in France,11 which are similar demographically to nursing homes in Canada.13 We used logistic regression to examine the individual predictors of IgG antibody status, symptoms, or asymptomatic infections, using Stata 16.14 The Ab-C study is approved by the Unity Health Toronto Ethics Review Board.
    Randomizationnot detected.
    Blindingnot detected.
    Power Analysisnot detected.
    Sex as a biological variablenot detected.

    Table 2: Resources

    No key resources detected.

    Results from OddPub: We did not detect open data. We also did not detect open code. Researchers are encouraged to share open data when possible (see Nature blog).

    Results from LimitationRecognizer: We detected the following sentences addressing limitations in the study:
    However, the study has some limitations. First, while our assay results were likely to be consistent throughout the Ab-C study population, the results are less comparable to seroprevalence studies using other assays. Ideally, testing with multiple assays should be done to reduce false positives in particular at low prevalence. However, since nearly all SARS-CoV-2 assays are recent, with less than a year for development and deployment, cross-comparisons that allow multiple assays are challenging.35 To improve comparisons over time and across assays, we recommend that national authorities organize a testing scheme with defined, blinded sample panels that can be provided to relevant laboratories. A similar strategy was successful at improving HIV diagnostics.36 Second, we could not directly measure seropositivity in residents in nursing homes, who accounted for most COVID deaths in Canada. By necessity, we used IFRs from French nursing homes, which were broadly similar in their demographics.27 The mortality-based and direct seroprevalence studies may not be comparable. Third, we may have overestimated the proportion of respondents who were asymptomatic, particularly at older ages, as we elicited a limited range of symptoms. The asymptomatic proportion of a quarter among definite seropositives was similar to that reported in England,23 higher than in a systematic review,37 and consistent with another that found more asymptomatic infections at older ages.38 Finally, our sample enr...

    Results from TrialIdentifier: No clinical trial numbers were referenced.

    Results from Barzooka: We did not find any issues relating to the usage of bar graphs.

    Results from JetFighter: We did not find any issues relating to colormaps.

    Results from rtransparent:
    • Thank you for including a conflict of interest statement. Authors are encouraged to include this statement when submitting to a journal.
    • Thank you for including a funding statement. Authors are encouraged to include this statement when submitting to a journal.
    • No protocol registration statement was detected.

    About SciScore

    SciScore is an automated tool that is designed to assist expert reviewers by finding and presenting formulaic information scattered throughout a paper in a standard, easy to digest format. SciScore checks for the presence and correctness of RRIDs (research resource identifiers), and for rigor criteria such as sex and investigator blinding. For details on the theoretical underpinning of rigor criteria and the tools shown here, including references cited, please follow this link.

  2. Version published to 10.1101/2021.03.04.21252540v1 on medRxiv