Due to the significant rise in the number of infections during the Omicron period and the widespread adoption of vaccines, a greater proportion of Canadians now have hybrid immunity – defined as immunity acquired from both prior infections and vaccination. Emerging evidence point to an added boost in immunity from infections among vaccinated individuals, including protection from subsequent infection. In a cohort study of 6 million Ontario residents, CITF-funded experts Drs. Jeffrey Kwong and Sharmistha Mishra (University of Toronto) found that a previous infection was associated with increased protection in individuals with either two doses or three doses when compared to a control group that had never been infected:

  • In individuals with two doses of vaccine, a previous infection was associated with a 68% increase in protection against an Omicron infection compared to infection-naïve individuals.
  • In individuals with three doses of vaccine, a previous infection was associated with a 43% increased protection against an Omicron infection compared to infection-naïve individuals.

It is important to note that the analyses of this study focused on relative protection and quantified additional protection among previously infected and vaccinated populations compared to vaccinated populations that had never been infected with SARS-CoV-2. The extent of added protection from infection-acquired immunity varies between populations with different vaccine dosages. In addition, the comparison groups in this study may have had residual protection from vaccines. However, in both cases, hybrid immunity conferred higher levels of protection than vaccination alone.

Ontario residents covered by the Ontario Health Insurance Plan were selected for inclusion in the study based on at least one reverse-transcription polymerase chain reaction (RT-PCR) test for SARS-CoV-2 between January 15, 2020, and November 21, 2021. Each individual was then followed from November 22, 2021, when the first Omicron case was identified in Ontario, until the end of the study period (December 31, 2021) or the primary outcome – Omicron infection (indicated by a positive SARS-CoV-2 PCR test) at least 90 days after a previous infection.

Wu S, Li Y, Mishra S, Bodner K, Baral S, Kwong JC, Wei X. Effect of the incremental protection of previous infection against Omicron infection among individuals with a hybrid of infection- and vaccine-induced immunity: a population-based cohort study in Canada. Int J Infect Dis. 2022 Nov 28;127:69-76. doi: https://doi.org/10.1016/j.ijid.2022.11.028.