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. Effectiveness of COVID-19 vaccines against Omicron or Delta symptomatic infection and severe outcomes. medRxiv 2021. Doi:

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

In a not yet peer-reviewed pre-print study led by CITF-funded researcher Dr. Jeff Kwong on behalf of the Canadian Immunization Research Network (CIRN), researchers evaluated the effectiveness of mRNA (or combined mRNA and AstraZeneca’s Vaxzevria) vaccines in preventing symptomatic infection and severe outcomes caused by the Omicron variant in Ontario. They found that three doses of a vaccine conferred excellent vaccine effectiveness (95% or more) against hospitalization or death for both the Delta and Omicron variants. When it came to vaccine effectiveness against symptomatic infection due to Omicron, while two doses of COVID-19 vaccines were initially 36% effective, a third dose increased vaccine effectiveness to 61%.

Key findings:

  • Seven days after a third, booster dose, vaccine effectiveness against severe outcomes (hospitalization or death) was similar for Delta (99%) and Omicron (95%).
  • After two-doses, vaccine effectiveness against symptomatic infection by the Delta variant declined steadily over time but recovered to 97% at least seven days following a third vaccine dose.
  • The protection against symptomatic infection by Omicron following two vaccine doses was lower and declined at a faster rate than protection against Delta.
  • Vaccine effectiveness against symptomatic Omicron infection improved from 37% to 61% seven days after receiving a third dose of an mRNA vaccine.

These results suggest that two doses of COVID-19 vaccines provide poorer protection against symptomatic infection from Omicron than Delta. However, when assessing severe outcomes such as hospitalization or death, vaccine effectiveness after a third vaccine dose was similar for Omicron and Delta infections.

The authors conclude that a third dose is needed to combat Omicron, in concert with other tools such as public health measures and the emerging antiviral therapies.

This study included symptomatic individuals over the age of 18 years in Ontario who took a reverse transcription real-time polymerase chain reaction (PCR) test for SARS-CoV-2 between December 6 and 26, 2021. A total of 16,087 Omicron-positive cases, 4,261 Delta-positive cases, and 114,087 test-negative controls were included in this analysis.

Vaccine effectiveness against symptomatic infection or severe outcomes (hospitalization, death) was assessed for Omicron or Delta variants for this period. Individuals who had received at least two COVID-19 vaccine doses (with at least 1 mRNA vaccine dose as part of the primary series) were included. For participants with a positive PCR test, researchers also identified whether the infection was due to the Omicron or Delta variant.