Factors associated with severe COVID-19 outcomes among adults with at least a primary vaccination schedule: A retrospective cohort study from Alberta, Canada.

BACKGROUND

With a large proportion of adults in Canada vaccinated against SARS-CoV-2, it is important to identify who remain at risk for severe COVID-19 outcomes after primary vaccination and additional (booster) doses.

METHODS

Adults (≥18 years) who received at least a primary vaccination schedule between December 2020 and March 2023 in Alberta, Canada were identified through population-level administrative data. Factors associated with severe COVID-19 outcomes (COVID-19 related hospitalization or death) were examined using univariable and multivariable Cox proportional hazard models.

RESULTS

Among those who received a primary vaccination schedule (n = 2,639,731), 0.2% (n = 5,475) had ≥ 1 severe COVID-19 outcome. Factors associated with a greater risk of a severe COVID-19 outcome included older age (≥65 versus <65 years; unadjusted hazard ratio [95% confidence interval]: 8.83 [8.33-9.35]), and living with a higher comorbid burden (severe versus no burden; 32.56 [29.78-35.60]) or health condition of interest (1.33 [1.25-1.42] to 29.25 [23.14-36.99]) such as the relatively common condition of cardiovascular disease (7.01 [6.62-7.42]; present in 10.5% of the cohort) and less common condition of dementia (13.96 [12.76-15.28]; present in 1.5% of the cohort); male sex was also associated with risk (versus female; 1.07 [1.01-1.13]). Results remained significant in fully adjusted analysis, and among those with ≥1 additional vaccination dose (n = 1,580,110).

CONCLUSION

Although primary vaccination and additional doses have dramatically reduced the occurrence of severe COVID-19 outcomes, results showed that continued risk remained for specific groups. Findings can be used to inform decision making, public health strategies, and policy development to protect those who remain at risk of severe COVID-19 outcomes after receipt of a primary vaccination schedule or additional doses, and support Canada's National Advisory Committee on Immunisation recommendations for specific groups to stay up to date with COVID-19 vaccination to protect against severe COVID-19 outcomes.