• Danuta M Skowronski, Samantha E Kaweski, Michael A Irvine, Erica S Y Chuang, Shinhye Kim, Suzana Sabaiduc, Romina C Reyes, Bonnie Henry, Inna Sekirov, and Kate Smolina.
• Immunization Programs and Vaccine Preventable Diseases Service (Skowronski, Kaweski, Chuang, Kim), BC Centre for Disease Control; School of Population and Public Health (Skowronski, Henry, Smolina), University of British Columbia; Data and Analytic Services (Irvine, Smolina), BC Centre for Disease Control, Vancouver, BC; Faculty of Health Sciences (Irvine), Simon Fraser University, Burnaby, BC; Public Health Laboratory (Sabaiduc, Sekirov), BC Centre for Disease Control; Department of Pathology and Laboratory Medicine (Reyes, Sekirov), University of British Columbia, Vancouver, BC; LifeLabs (Reyes), Burnaby, BC; Ministry of Health (Henry), Office of the Provincial Health Officer, Victoria, BC danuta.skowronski@bccdc.ca.
• CMAJ. 2023 Oct 30; 195 (42): E1427E1439E1427-E1439.

BackgroundPopulation-based cross-sectional serosurveys within the Lower Mainland, British Columbia, Canada, showed about 10%, 40% and 60% of residents were infected with SARS-CoV-2 by the sixth (September 2021), seventh (March 2022) and eighth (July 2022) serosurveys. We conducted the ninth (December 2022) and tenth (July 2023) serosurveys and sought to assess risk of severe outcomes from a first-ever SARS-CoV-2 infection during intersurvey periods.MethodsUsing increments in cumulative infection-induced seroprevalence, population census, discharge abstract and vital statistics data sets, we estimated infection hospitalization and fatality ratios (IHRs and IFRs) by age and sex for the sixth to seventh (Delta/Omicron-BA.1), seventh to eighth (Omicron-BA.2/BA.5) and eighth to ninth (Omicron-BA.5/BQ.1) intersurvey periods. As derived, IHR and IFR estimates represent the risk of severe outcome from a first-ever SARS-CoV-2 infection acquired during the specified intersurvey period.ResultsThe cumulative infection-induced seroprevalence was 74% by December 2022 and 79% by July 2023, exceeding 80% among adults younger than 50 years but remaining less than 60% among those aged 80 years and older. Period-specific IHR and IFR estimates were consistently less than 0.3% and 0.1% overall. By age group, IHR and IFR estimates were less than 1.0% and up to 0.1%, respectively, except among adults aged 70-79 years during the sixth to seventh intersurvey period (IHR 3.3% and IFR 1.0%) and among those aged 80 years and older during all periods (IHR 4.7%, 2.2% and 3.5%; IFR 3.3%, 0.6% and 1.3% during the sixth to seventh, seventh to eighth and eighth to ninth periods, respectively). The risk of severe outcome followed a J-shaped age pattern. During the eighth to ninth period, we estimated about 1 hospital admission for COVID-19 per 300 newly infected children younger than 5 years versus about 1 per 30 newly infected adults aged 80 years and older, with no deaths from COVID-19 among children but about 1 death per 80 newly infected adults aged 80 years and older during that period.InterpretationBy July 2023, we estimated about 80% of residents in the Lower Mainland, BC, had been infected with SARS-CoV-2 overall, with low risk of hospital admission or death; about 40% of the oldest adults, however, remained uninfected and at highest risk of a severe outcome. First infections among older adults may still contribute substantial burden from COVID-19, reinforcing the need to continue to prioritize this age group for vaccination and to consider them in health care system planning.© 2023 CMA Impact Inc. or its licensors.

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