A recent surge in infections caused by the Omicron variant during the last respiratory virus season has resulted in a significant portion of Canada’s population gaining immunity against SARS-CoV-2, responsible for COVID-19. These statistics, highlighting immunity acquired through infections rather than vaccinations, were generated by the COVID-19 Immunity Task Force. Published in the Canadian Medical Association Journal on Monday, the findings indicated that more than 75% of individuals had been infected by March of this year due to the prolonged dominance of the Omicron strain over 16 months.
Infection-driven immunity surpasses vaccination efforts: Dr. Bruce Mazer, co-lead of the study and a senior scientist at the McGill University Health Centre research institute, explained, “Despite high vaccine coverage in Canada, the infection rate rose rapidly with the highly contagious Omicron variant.” He highlighted the significant impact of the Omicron variant in driving infections, leading to a substantial increase in acquired immunity among the population.
Omicron’s rapid spread leads to drastic increase in infection-acquired immunity: As the Omicron variant circulated through Canada for six months, reaching its peak in mid-June 2022, infection-acquired immunity surged to 47 percent. This trend persisted with an average monthly increase of 6.4 percent, culminating in over 75 percent by March 2023. This stark contrast to earlier pandemic stages, where minimal evidence of previous infections was found in blood samples, highlighted the transformative impact of the Omicron variant.
Shift in demographics and geographical trends: The study’s co-lead, David Buckeridge, a professor of global health at McGill, highlighted the distinct changes in infection rates during the Omicron wave. “During Omicron, rates of infection-acquired immunity increased faster in younger age groups and the western provinces of Manitoba, Saskatchewan, Alberta, and British Columbia,” stated Buckeridge. By March 2023, evidence of a previous infection was present in around 80 percent of adults aged 18-25, compared to approximately 75 percent aged 25-39, 70 percent aged 40-59, and 60 percent aged 60 and above.
Considerations for future public health policy: The data on infection rates and acquired immunity is critical for formulating future public health policies concerning COVID-19. Notably, it emphasizes the need to monitor age, geographic variations, and potential fluctuations in antibody levels over time. To gather this insightful information, the task force collaborated with seven teams of scientists, utilizing aggregate data to discern trends in infection-acquired and vaccine-induced SARS-CoV-2 presence in blood samples.
Phases of the pandemic analyzed: The statistics presented by the task force are drawn from three distinct phases of the pandemic:
- The pre-vaccination period from March to November 2020
- The vaccine rollout period from December 2020 to November 2021
- The Omicron waves spanning from December 2021 to March 2023
The task force, funded by the Public Health Agency of Canada, has supported 120 research studies dedicated to understanding immunity to SARS-CoV-2, particularly among underrepresented populations and those in long-term care settings.