Hospitalization and mortality risks from COVID-19 by age during SARS-CoV-2 Delta and Omicron variants’ predominance

In a recent study published in the Canadian Medical Association Journal, researchers evaluated age-stratified hospitalization and mortality risks from incident severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections in British Columbia (BC), Canada, during the SARS-CoV-2 Delta variant of concern (VOC) and Omicron VOC predominance.

Study: 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. Image Credit: FamVeld/Shutterstock/com


Studies have reported that individuals with a prior history of SARS-CoV-2 vaccination and infection have a reduced risk of coronavirus disease 2019 (COVID-19) severity outcomes compared to those without exposure, and identifying the fraction of uninfected individuals is crucial for ongoing risk evaluation.

In the initial phase of COVID-19, male sex and older age have been reported as independent estimators of COVID-19 severity. Seroprevalence estimates help capture and quantify infections, but their generalizability depends on the sample population.

The British Columbia Centre for Disease Control (BCCDC) performed eight population-level, cross-sectional SARS-CoV-2 seroprevalence surveys from March 2020 to August 2022.

The surveys indicated that COVID-19 incidence was 10% during the sixth survey in September 2021, 40% during the seventh survey in March 2022, and 60% during the eighth survey in July 2022.

About the study

In the present study, researchers conducted serosurveys 9.0 in December 2022, followed by serosurvey 10 in July 2023, respectively, to assess changes in SARS-CoV-2 seroprevalence, particularly in the elderly, and evaluate severe COVID-19-related outcome risk from incident COVID-19 during the inter-survey periods.

Cumulative COVID-19-induced seroprevalence, severe outcomes, population count, discharge abstracts, and vital statistical data were used to estimate infection hospitalization ratios (IFRs) and infection fatality ratios (IFRs) by gender and age during the period between serosurveys 6.0 and 7.0 (Delta VOC/Omicron VOC’s BA.1 sub-VOC), serosurveys 7.0 and 8.0 (BA.2 sub-VOC/BA.5 sub-VOC), and serosurveys 8.0 and 9.0 (BA.5 sub-VOC/BQ.1 sub-VOC) inter-survey periods.

The derived IHRs and IFRs represented severe COVID-19-related outcome risk from incident infections during the predetermined inter-survey periods. COVID-19 was confirmed using nucleic acid amplification tests (NAATs). The sample population included individuals presenting for blood draws at the LifeLabs diagnostic outpatient center.

The LifeLabs Center provided BCCDC researchers with sera from 2,000 anonymized BC residents, including 200 serum samples for all age groups (zero to four years, five to nine years, and 10-year categories through 80 years and older).

Samples obtained for COVID-19 testing from long-term care recipients, individuals with assisted living, and prisoners were excluded.

Antibodies against the SARS-CoV-2 spike protein subunit 1 (S1) and nucleocapsid (NP) protein were detected using chemiluminescent immunoassays. Non-orthogonal testing was performed in serosurveys 9.0 and 10, and observations from serosurveys 6.0 to 8.0 were similarly re-analyzed.

Bayesian analysis was performed to estimate seroprevalence, adjusting for gender, health authorities, and age.

On August 24, 2023, the team extracted COVID-19 severity outcome data from the British Columbia coronavirus disease 2019 cohort (BCC19C), including the discharge abstract database (DAD), the provincial vital statistical database, and the British Columbia Centre for Disease Control integrated surveillance data for cases of COVID-19 confirmed by NAAT and the International Classification of Diseases, 10th Revision, Canadian version (ICD-10-CA) codes.


The median participant age was 40 years, and 50% were female. The cumulative SARS-CoV-2 seroprevalence rate through December 2022 was 74%, and through July 2023 was 79%, surpassing 80% among individuals below 50 but persisting below 60% among individuals aged ≥80 years.

Period-specific infection hospitalization and fatality ratios remained consistently under 0.30% and 0.10%, respectively.

Age-stratified infection hospitalization and fatality ratios were mostly below one percent and ≤0.1%. However, there were exceptions. Individuals aged between 70 and 79 years in the period between serosurveys 6.9 and 7.0 had an IHR and IFR of three percent and one percent, respectively.

Among the elderly aged ≥80 years during all inter-survey periods, IHRs were five percent, two percent, and four percent. IFRs for this age group were three percent, one percent, and one percent in the periods between serosurveys 6.0 and 7.0, 7.0 and 8.0, and 8.0 and 9.0, respectively. The pattern for severe COVID-19 outcome risks by age was J-shaped.

In the period between serosurveys 8.0 and 9.0, the team estimated one COVID-19-related hospitalization per 300 children aged below five years with incident COVID-19 vs. one hospitalization per 30 adult individuals aged ≥80 years with incident infection, with no COVID-19-related mortality in children but one death among every 80 adults with incident infection among individuals aged ≥80 years during the period.

The exploratory analyses showed some gradation in the risks of hospitalization and mortality per incident infection between individuals aged 60 to 64 years (one per 1,400 and 10,000, respectively) vs. 65 to 69 years (one per 500 and 2500, respectively).


Based on the study findings, through July 2023, the researchers estimated that 80% of BC residents were infected by SARS-CoV-2, with low risks of hospitalization or death from COVID-19 in the context of high vaccine coverage contributing to hybrid protection.

However, 40% of elderly individuals did not develop the SARS-CoV-2 infection but had a heightened severe outcome risk.

The findings indicated that incident infections among elder individuals might contribute considerably to the COVID-19 burden on healthcare systems, highlighting that health authorities must continue prioritizing the elderly for COVID-19 vaccinations and consider them during healthcare planning.

Journal reference:
  • Danuta M. Skowronski MD et al., (2023) 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, CMAJ 2023, October 30;195:E1427-39, doi: 10.1503/cmaj.230721.

Posted in: Medical Science News | Medical Research News | Disease/Infection News

Tags: Antibodies, Blood, Children, Coronavirus, Coronavirus Disease COVID-19, covid-19, Diagnostic, Healthcare, Immunoassays, International Classification of Diseases, Mortality, Nucleic Acid, Omicron, Protein, Respiratory, SARS, SARS-CoV-2, Severe Acute Respiratory, Severe Acute Respiratory Syndrome, Spike Protein, Syndrome, Vaccine

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Pooja Toshniwal Paharia

Dr. based clinical-radiological diagnosis and management of oral lesions and conditions and associated maxillofacial disorders.