Background: Efforts to stem the SARS-CoV-2 pandemic in Canada 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.

中文翻译：

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加拿大第一波SARS-CoV-2流行期间的COVID血清阳性率，症状和死亡率

背景：在加拿大遏制SARS-CoV-2大流行的努力可以受益于对流行率，感染死亡率（IFR）和无症状感染信息的直接了解。方法：我们对19,994名加拿大成年人的代表性样本进行了COVID症状调查，并从8,967名成年人的自收集干血斑（DBS）中分析了针对SARS-CoV-2的IgG抗体。灵敏而特异的化学发光ELISA检测到加标三聚体的IgG。我们将血清流行率与死亡进行了比较，以建立IFR，并使用死亡率数据来评估疗养院居民的感染水平。结果：全国成人血清流行率的最佳估计值（高特异性）为1.7％，但取决于测定的临界值，则高达3.5％（高灵敏度）。患病率最高的是安大略省（2.4-3。9％）和年龄在18-39岁之间的年轻人（2.5-4.4％）。根据死亡率，我们估计有13-17％的疗养院居民被感染。第一次病毒波感染了504.108万成年加拿大成年人，其中一半年龄小于40岁。养老院外的IFR为0.20-0.40％，但养老院居民的COVID死亡率比社区中同龄成年人的COVID死亡率高70倍以上。血清阳性与COVID症状相关，尤其是在三月份。无症状的成年人约占血清阳性的四分之一，在老年人中占更大的比例。解释：在第一波病毒浪潮中，加拿大社区（而非养老院）的感染率和IFR相对较低。