Mr. Speaker, in answer to part (a) of the question, at the time of initial authorization of the first COVID-19 vaccines, Pfizer-BioNTech, in December 2020, there was no reported evidence on the efficacy of the authorized COVID-19 vaccine to prevent asymptomatic infection, to reduce viral shedding or to prevent transmission. In February and March 2021, preliminary data from the ongoing vaccine trials showed a lower prevalence of SARS-CoV-2 infection in asymptomatic participants in the short term.
In December 2021, in the context of the circulating delta, or B.1.617.2, variant, evidence was emerging that vaccine effectiveness against SARS-CoV-2 infection and COVID-19 decreases with time after the primary series and there may be some decrease in protection against severe illness, especially in older individuals. Decreasing protection against infection could contribute to increased transmission, since infected individuals may be a source of infection for others. Therefore, it was determined that a booster dose may provide more durable protection to reduce infection, transmission and, in some populations, severe disease.
All evidence used to inform COVID-19 vaccine recommendations is accessible in the publicly available NACI statements.
In response to part (b) of the question, in April 2020 the Government of Canada set up the COVID-19 immunity task force, or CITF, as a time-limited task force charged in part to determine the extent of SARS-CoV-2 infection and immune response in the Canadian population and specific high-risk subgroups. Over its term, the CITF has mobilized critical science to provide evidence for decisions around management of the COVID-19 pandemic, including but not limited to seroprevalence data on population immunity.
The CITF has continued to monitor the seroprevalence of the Canadian population for the last four years, drawing on serosurveillance studies that measured antibodies due to infection, vaccination and/or a combination of the two. The CITF has maintained an up-to-date, publicly accessible website with trends in seroprevalence over the course of the pandemic and has been providing monthly updates to the Public Health Agency of Canada. The funding and policy authorities for this initiative end on March 31, 2024.
In response to part (c) of the question, naturally acquired immunity alone can protect against infection in the short term, but less is known about the long term. In addition, immunity through prior infection can wane over time and may not provide protection against infection or illness if the strain causing a previous infection is different from the strains currently circulating. Studies have also shown that the level of immune responses due to infection alone can vary significantly between individuals and that reinfection is more likely to happen in people who are unvaccinated compared to those who have been vaccinated.
COVID-19 vaccines provide enhanced protection against symptomatic disease, particularly severe disease. Protection against infection wanes over time for those who are vaccinated as well. However, protection is more sustained against severe COVID-19 illness. COVID-19 vaccines have been updated to target more recent strains and continue to show good immune responses to currently circulating strains. In addition, there are no known safety risks with receiving a vaccine after a recent SARS-CoV-2 infection. Prior infection along with vaccination, known as hybrid immunity, offers greater protection against infection and severe disease than vaccination or prior infection alone, particularly when hybrid immunity is in the context of a recent omicron infection.