Effectiveness of COVID-19 vaccines against symptomatic SARS-CoV-2 infection and severe outcomes with variants of concern in Ontario.
Level 4 - case-series / case-control
Observational test-negative case-control design
PubMed 35132198 · doi:10.1038/s41564-021-01053-0
What was done
A test-negative design study evaluated the real-world effectiveness of BNT162b2 (Pfizer-BioNTech), mRNA-1273 (Moderna), and ChAdOx1 (AstraZeneca) vaccines against symptomatic SARS-CoV-2 infection and severe outcomes (hospitalization or death) caused by Alpha, Beta, Gamma, and Delta variants in Ontario, Canada. The sample included 682,071 symptomatic community-dwelling individuals tested for SARS-CoV-2 and 15,269 individuals hospitalized with or dying from COVID-19.
What was found
At ≥7 days after receiving two doses, vaccine effectiveness against symptomatic infection ranged across vaccine products from 89% to 92% for Alpha, 87% for Beta, 88% for Gamma, 82% to 89% for combined Beta/Gamma, and 87% to 95% for Delta. Single-dose effectiveness (≥14 days post-dose) was consistently lower. Protection against hospitalization or death was comparable to or higher than protection against symptomatic infection. Vaccine effectiveness against symptomatic infection was generally lower in adults aged ≥60 years than in those <60 years across most variant-vaccine combinations.
Why it matters
This study confirms that completing a two-dose primary series of mRNA or adenoviral vector vaccines provided substantial protection against severe disease and symptomatic illness across pre-Omicron variants of concern, while highlighting lower protection in older adults when doses are delayed.
Limits
The test-negative observational design is vulnerable to unmeasured confounding, including differential exposure risks or testing behaviors. The abstract does not report specific 95% confidence intervals, product-specific estimates, or long-term durability of protection, and the analysis predates the emergence of the Omicron lineage.
Cited by
- supports Against the Alpha and Beta variants of SARS-CoV-2, COVID-19 vaccines used in the United States were 90% to 95% effective at preventing symptomatic infection.