Risk of hospital admission and death from first-ever SARS-CoV-2 infection by age group during the Delta and Omicron periods in British Columbia, Canada

Abstract

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