Effectiveness vs. Efficacy: What’s the Difference Anyway?

“Effectiveness” vs. “Efficacy.” Is there a difference?

By Howard Wolinsky

Stefanie Demetriades, PhD, a health communication researcher for CoVAXCEN, a coalition of academic, industry, and government agencies, said the public may consider the terms interchangeable. But she said they have different uses.

The term efficacious refers to results in highly controlled clinical trials, while the term effective measures the vaccines’ impact in less predictable real-world vaccinations, she said.

Demetriades said understanding how to interpret these measures will impact public understanding of the benefits and risks of the vaccines and whether the public opts to be immunized.

When the general public hears that the Pfizer vaccine is 95% efficacious, it may take it to mean that 5% of patients got infected with COVID-19.

        “But that’s not accurate,” said Demetriades.

        The percentage who contracted COVID actually is 0.04%, 100 times less, she said.

She said the 95% measure means that people vaccinated with the Pfizer vaccine had a 95% reduced risk of contracting COVID-19 than unvaccinated subjects. Vaccinated people in the Pfizer clinical trial were 20 times less likely than the control group to get COVID-19.

The evidence is that the all three vaccines are both highly efficacious in clinical trials and highly effective in real world conditions, with across-the-board success in protecting against serious illness and death from COVID-19.