In this not-yet peer reviewed pre-print, led by CITF-funded researcher Dr. Jeff Kwong on behalf of the Canadian Immunization Research Network (CIRN), researchers evaluated the effectiveness of mRNA vaccines against the Omicron variant in Ontario. They found that two doses of COVID-19 vaccines were not protective against Omicron infection. However, more than seven days after a third dose of an mRNA vaccine, effectiveness against Omicron infection was 37%.

Key findings:

  • After a two-dose regimen, vaccine effectiveness against infection by the Delta variant declined steadily over time, but recovered to 93% more than 7 days after receiving a third dose mRNA vaccine booster.
  • Receiving two doses of COVID-19 vaccines was not protective against Omicron at any point in time. Vaccine effectiveness against Omicron infection was 37% more than 7 days after receiving an mRNA vaccine for a third dose.
  • Findings were consistent for any combination of two mRNA vaccines and two doses of Comirnaty (Pfizer) for the primary series.

These results suggest that two doses of COVID-19 vaccines are not enough to confer protection against Omicron, although they are effective against Delta. There is a significant increase in effectiveness after a third dose. Importantly, the study did not measure symptoms or severity, but only whether the subject was infected.

The authors conclude that additional tools, such as public health measures and antivirals, are likely needed to protect against Omicron infection, in combination with vaccination going forward.

This study included all individuals over the age of 18 years in Ontario with provincial health insurance who took a reverse transcription real-time polymerase chain reaction (PCR) test for SARS-CoV-2 between November 22 and December 19, 2021. Vaccine effectiveness against infection 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 for the primary series) were included. For participants with a positive PCR test, researchers went on to identify whether the infection was due to the Omicron or Delta variant.

A total of 3,442 Omicron-positive cases, 9,201 Delta-positive cases, and 471,545 test-negative controls were included in this analysis.

Buchan SA, Chung H, Brown KA, Austin PC, Fell DB, Gubbay J, Nasreen S, Schwartz KL, Sundaram ME, Tadrous M, Wilson K, Wilson SE, Kwong J. Effectiveness of COVID-19 vaccines against Omicron or Delta infection. MedRxiv 2021. doi: