This is a summary, written by members of the CITF Secretariat, of:

Buchan SA, Chung H, Brown KA, Austin PC, Fell DB, Gubbay JB, Nasreen S, Schwartz KL, Sundaram ME, Tadrous M, Wilson K, Wilson SE, Kwong JC. Estimated Effectiveness of COVID-19 Vaccines Against Omicron or Delta Symptomatic Infection and Severe Outcomes. JAMA Network Open. 2022 Sep 22. doi:

The results and/or conclusions contained in the research do not necessarily reflect the views of all CITF members.

A CITF-funded study led by Dr. Jeff Kwong (University of Toronto), published in JAMA Network Open, estimated that two doses of any COVID-19 vaccine were highly effective against a symptomatic Delta infection, but efficacy was lower and protection was more short-lived against a symptomatic Omicron infection. Vaccine effectiveness (VE) against severe outcomes was high after the third dose of an mRNA vaccine for both Delta and Omicron.

Key findings:

  • Estimated VE against symptomatic Delta infection was 89% (7 to 59 days after the second dose) and decreased to 80% 240 days after dose two. The VE increased to 97%, 7 or more days after a third dose.
  • Estimated VE against symptomatic Omicron infection was 36% (7 to 59 days after the second dose), waning rapidly to 1% between 180 to 239 days. The VE increased to 61% 7 or more days after a third dose.
  • Estimated VE against severe outcomes for both Delta (99%) and Omicron (95%) was high 7 days after a third dose.
  • Compared to controls, Omicron cases were most frequent among those who were:
    • younger
    • male
    • less likely to have any comorbidities
    • less likely to have been vaccinated against influenza
    • less likely to have been previously tested positive for SARS-CoV-2
    • less likely to have been vaccinated with a third dose, and
    • most likely infected during the last week of the study period.
  • People who contracted the Delta variant were more likely to be unvaccinated.

The results of this study suggest that a third vaccine dose is important to bolster protection against both Omicron infection and severe outcomes.

This is test-negative case-control study that utilized linked databases in Ontario for SARS-CoV-2 laboratory testing, reportable disease, COVID-19 vaccination, and health administration. Participants included in the study were 18 years of age or older who had COVID-19 symptoms or severe outcomes (hospitalization or death) and were tested for SARS-CoV-2 between December 6 and 26, 2021. They received at least 1 mRNA vaccine for the primary 2-dose series. The study population included 134,435 adults, of whom 16,087 (12%) were Omicron-positive cases, 4,261 (3%) were Delta-positive cases, and 114,087 (85%) were test-negative controls.