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-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.

Methods: Using 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.

Results: The 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.

Interpretation: By 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.

Figure 1:

Provincial surveillance case reports by epidemiological week and timing of serosurveys, British Columbia, Canada, January 2020 (epidemiological week 3) to August 2023 (epidemiological week 32). Weekly surveillance case reports of SARS-CoV-2 infections confirmed by nucleic acid amplification test (NAAT) were reported to the British Columbia Centre for Disease Control (BCCDC) from the Fraser Health Authority (HA) and Vancouver Coastal HA in the Lower Mainland, as well as other provincial HAs (combined). Case tallies are grouped by epidemiological week (7-d period) as per standard surveillance methods for comparing data by period from year to year. Serosurveys exclude those identified as assisted living, independent living or long-term care facility residents but provincial case tallies do not apply those exclusions. Epidemic waves are indicated with the predominant variant of concern (VOC). Changes in publicly funded access to NAATs or rapid antigen tests (RATs) are displayed below the x-axis.

Figure 2:

Cumulative vaccine- and infection-induced SARS-CoV-2 seroprevalence by age group, sixth to tenth serosurveys, in the Lower Mainland, British Columbia, Canada (September 2021–July 2023). (A) Side-by-side comparison of the sixth to ninth serosurveys to illustrate seroprevalence progression. (B) Ninth and tenth serosurveys, presented separately for comparison of recent age-related patterns. Detailed findings are provided in Table 2. Darker bars indicate infection-induced seroprevalence. Lighter bars, combined with the darker bars, indicate overall (vaccine-induced, infection-induced or both) seroprevalence. Infection-induced estimates were defined by anti-nucleocapsid positivity. Overall estimates were defined by anti-spike or anti-nucleocapsid positivity. Estimates were based on Bayesian analyses, standardized for age, sex and health authority. Estimates from the sixth to eighth serosurveys are updated from our previous study, consistently applying the same nonorthogonal approach. Note: CrI = credible interval.

Figure 3:

Flowchart of hospital admissions for COVID-19 attributed to first-ever SARS-CoV-2 infection. We used the British Columbia COVID-19 Cohort. All data were extracted on Aug. 24, 2023. Where step-specific tallies of U07.1- and U07.3-coded hospital admissions do not sum to the displayed total, it is because both diagnostic codes were specified. In addition, but not displayed here, of 1346 deaths identified within the vital statistics database since Jan. 1, 2020, among Lower Mainland residents with underlying cause specified as U07.1 (none specified as U07.3) during the span of intersurvey periods, we excluded 59 (4.4%) with a SARS-CoV-2 NAAT-positive test 90 days or more before date of death. Fewer than 1% of hospital admissions identified provincially were missing information to assign health authority to the Lower Mainland. We did not exclude any hospital admissions or deaths on the basis of missing age or date of admission or death. We did not exclude any hospital admissions on the basis of missing sex, and excluded fewer than 10 fatalities on this basis, handled as indicated in Appendix 1, Supplementary Material 1. Note: FHA = Fraser Health Authority, NAAT = nucleic acid amplification test, VCHA = Vancouver Coastal Health Authority.

Figure 4:

Estimated risk of hospital admission and death from first-ever SARS-CoV-2 infection acquired during the specified intersurvey period, by age group, Lower Mainland, British Columbia, Canada. (A) Period-specific infection hospitalization ratios (IHRs) and (B) infection fatality ratio (IFRs) by age group. Panels A and B do not show 95% credible intervals (CrIs) (but are provided in Table 3) for better resolution, given that upper CrIs extended past 10%. (C) Period-specific IHRs (log₁₀ scale) and (D) IFRs (log₁₀ scale) with 95% CrIs.

Figure 5:

Estimated period-specific risk of hospital admission and death from first-ever SARS-CoV-2 infection by sex, overall and by age group, Lower Mainland, British Columbia, Canada. (A) Period-specific infection hospitalization ratios (IHRs) and (B) infection fatality ratio (IFRs) with their 95% credible intervals (CrIs), using log₁₀ scale. Because of fewer severe outcomes, we did not stratify IHR and IFR estimates by sex for separate age groups younger than 50 years. All ages refers to all age groups combined from younger than 5 years to 80 years and older. Details are provided in Appendix 1, Supplementary Tables 11–13.