Apples to Oranges: COVID-19 Vaccine Efficacy


With all the media coverage surrounding the Health Canada-approved COVID-19 vaccines, it’s time we truly understand what vaccine efficacy means to Canadians. The Moderna vaccine has an efficacy rate of 94%, the Pfizer-BioNtech vaccine 95%, the AstraZeneca vaccine 76% and the Johnson & Johnson vaccine 66%. You’ve likely heard these figures discussed on the news, and you may be tempted to compare the rates against one another. In reality, it’s not that simple and it’s not good science.


Efficacy is the degree to which a medication, or in this case a vaccine, prevents a disease in clinical trials. Efficacy does not directly translate to effectiveness, or how well we can expect the vaccine to perform in a real-world setting. For instance, if the Pfizer-BioNtech vaccine has an efficacy of 95%, this means in clinical trials there was a 95% reduction in the number of COVID cases in the vaccinated group, compared to the placebo group. Likely, this percentage would be quite different in the general population.


Clinical trial efficacy for the COVID-19 vaccines depends on many factors. For example, these vaccines were studied at different timepoints, in different populations of individuals. This is important because when we consider the emergence of highly infectious and more severe variants in the later stages of the pandemic, vaccines such as the Johnson & Johnson studied later on, may seemingly underperform in comparison to other vaccines studied earlier in the evolution of the virus.


Realistically, we don’t know how the vaccines would stack up against each other unless they were studied in controlled, head-to-head clinical trials. What we do know however, is that every Health Canada-approved COVID-19 vaccine is 100% effective in preventing hospitalizations and death and that is what is most important to the health of Canadians.

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