About 80% of residents in the Lower Mainland, British Columbia, had been infected with SARS-CoV-2 by July 2023 and were at low risk for severe outcomes, but almost half of the oldest adults remained uninfected and were at highest risk of hospitalization and death due to COVID-19, according to a new study published in CMAJ (Canadian Medical Association Journal).

The findings underscore the need to continue prioritizing older adults for COVID-19 vaccination.

"First-ever SARS-CoV-2 infections among older adults may still contribute substantial COVID-19 burden, reinforcing the importance of their continued prioritization for vaccination and their consideration in health care system planning," writes lead investigator Dr. Danuta Skowronski, BC Center for Disease Control and the University of British Columbia, Vancouver, BC, with co-authors.

Using population-based seroprevalence estimates of first-ever SARS-CoV-2 infection combined with administrative data on severe outcomes, researchers estimated the per infection risk of hospital admission and death. They found that by July 2023, more than 80% of children and adults younger than 50 had been infected and had a low risk of severe outcome, but more than 40% of adults aged 80 and older had never been infected with SARS-CoV-2 and had the highest risk of hospitalization and death.

The authors estimated that between July and December 2022 the risk of hospitalization for COVID-19 was about one in 30 new infections among adults 80 years and older, which was about 10 times higher than the risk among children younger than five years of age.

"Our estimates of the risk of hospital admission or death from a first-ever SARS-CoV-2 infection were low overall but derived in a highly vaccinated population. Risks are anticipated to be greater among unvaccinated and lower among previously infected groups of patients, with the lowest risk among those who are both vaccinated and previously infected," the authors write.