The incidence of COVID-19 infection and poor outcomes in the last half of 2021 was consistently lowest among Canadians who completed a primary vaccination series and highest among the unvaccinated, with the most severe consequences among those aged 80 and older of any vaccination status, the Public Health Agency of Canada (PHAC) reports.
The researchers mined the National COVID-19 Case Dataset and the Canadian COVID-19 Vaccination Coverage Surveillance System for data on people aged 12 years and older in 12 of 13 provinces and territories by vaccination status, COVID-19 outcomes, and demographic factors from December 2020 to January 2022.
Unvaccinated 11 to 17 times more likely to die
During the study period, 1,194,694 COVID-19 infections in patients with complete vaccination history (73.6% of all cases) were reported to PHAC. Most infected patients were unvaccinated, and the lowest percentage of infections was among those who received a primary COVID-19 vaccination series and one booster dose.
During the emergence of the SARS-CoV-2 Delta variant (May to July 2021), compared with vaccinated people, the unvaccinated were 6.8 times more likely to be infected, 11.4 times more likely to be hospitalized, and 17.5 times more likely to die. Amid Delta predominance (July to December 2021), those figures were 6.2, 21.0, and 15.4, respectively.
After the emergence of the Omicron variant (December 2021 to January 2022), the increased likelihoods for unvaccinated participants were 0.9, 7.1, and 11.3 for infection, hospitalization, and death, respectively.
Oldest age-groups most at risk for poor outcomes
"Case incidence in 2021 was consistently highest in unvaccinated individuals, with younger age groups having the highest incidence rates," the authors wrote.
"Starting in spring 2021, there was an increase in vaccine breakthrough cases, consistent with studies showing that, although completion of a primary vaccination series was highly effective in preventing infection against the wild-type virus and Alpha variant, it was slightly less effective against the Beta, Gamma, and Delta variants."
Vaccination remains one of the most important public health interventions, particularly among older adults, to protect against COVID-19 severe outcomes as the pandemic evolves.
Rates of poor outcomes were consistently highest among those aged 80 years and older, followed by those aged 60 to 79 for all vaccination statuses. Infection incidence was highest among adults aged 18 to 39, followed by those aged 40 to 59, from mid-2021 to late August 2021. Infection incidence rates of in these older age-groups fell after vaccine uptake rose, until Omicron emergence.
"Vaccination remains one of the most important public health interventions, particularly among older adults, to protect against COVID-19 severe outcomes as the pandemic evolves," the study authors wrote. "Routine monitoring of COVID-19 outcomes by vaccination status can identify changes in COVID-19 epidemiology and inform public health action and policy."
