A look at the difference between COVID-19 vaccine efficacy and effectiveness


As Canada approves more COVID-19 vaccines, health officials are advising to take the first one made available to you despite its efficacy rate.

Experts say a vaccine’s effectiveness and efficacy are two different things.

Efficacy refers to how well a drug, in this case, a vaccine, works under controlled circumstance such as a clinical trial. Effectiveness refers to how well that drug or vaccine works in the real world.

Dr. Matthew Miller, assistant dean at McMaster University’s department of biochemistry and biomedical sciences, said a lower efficacy rate does not always mean a low level of effectiveness in the real world and the same is true for vaccines of high efficacy.

He said the efficacy numbers for vaccines tend to be higher than effectiveness due to variables happening in the real world that can’t be accounted for during a clinical trial.

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Read more: Experts say lower vaccine efficacy shouldn’t be deterrent

Clinical trials are carefully designed to help mitigate inconsistencies between efficacy and effectiveness, he said. People with underlying health issues or certain types of medication are usually excluded from trials and any side effects found are closely monitored.

However, “in practice, we usually see that the effectiveness numbers are modestly lower than the efficacy numbers,” he said.

“It’s very rare that you would see a vaccine behave in a way where there’s really high efficacy in a clinical trial, but then much lower effectiveness.”

The recently approved AstraZeneca vaccine, for example, showed a 62 and 72 per cent efficacy. This means that 62 per cent and 72 per cent of participants in a vaccinated group saw a reduction in cases in a comparison with the unvaccinated (or placebo) group during their respective trials.

This differs from the near 95 per cent effectiveness of mRNA vaccines such as Pfizer/BioNtech and Moderna. However, all four vaccines authorized for use in Canada — Pfizer, Moderna, AstraZeneca and Johnson & Johnson — showed perfect data when it came to preventing death or hospitalization.

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Coronavirus: Health officials urge people to take whichever vaccine available regardless of efficacy rate – Mar 5, 2021

According to Miller, there is a common misconception that vaccines are always supposed to prevent someone from contracting a disease or virus, but that isn’t necessarily the immediate goal.

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“A vaccine that might not be great at preventing people from getting symptoms, but might be really good at ensuring that they don’t get severely ill,” he said.

Using the AstraZeneca vaccine as an example, Miller noted it may be “modestly inferior” to other mRNA vaccines in its ability to prevent people from getting any symptoms, “but it’s extraordinarily effective in making sure that even if they do get symptoms, that it’s just a mild illness.”

“The imperative right now is just to ensure that as many people are vaccinated so that they’re immune as soon as possible to reduce the number of severe illnesses and deaths, which is the outcome that we care most about,” he noted.

Read more: WHO says continue to use AstraZeneca vaccine, will investigate issues

Re-vaccination a possibility

According to global health partnership Gavi, The Vaccine Alliance, the effectiveness of a vaccine is measured in “observational studies” because “participants are not randomly assigned to a treatment versus a placebo group.”

On its website, Gavi writes that “case-control studies assess effectiveness by comparing the vaccination status of individuals who develop the disease (cases) with a group of individuals without the disease (controls) who are also representative of the population from which the cases arise.”

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“If the vaccine is effective, the cases are more likely to be the unvaccinated individuals.”

Moving into the future, Miller said Canadians can expect to see updated recommendations around if certain people need another booster later on and possibly even a boost with a different vaccine.

“Those are all scenarios that are being looked at,” he said.

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