Efficacy of the Covid-19 vaccine: what do the numbers really mean?

The spread of the Delta variant, an increase in the number of people vaccinated testing positive for Covid-19, and the U.S. government’s call for booster shots have raised new questions about the potency of coronavirus vaccines. Here’s what we know about the performance of licensed variant vaccines and their ability to prevent serious infections and illnesses.

What does it mean to say that a vaccine is effective?

Health experts and scientists use a variety of terms and measures to describe the effectiveness of a vaccine. Vaccines can be evaluated for their ability to prevent initial infection, symptomatic illness, or serious illness that can lead to hospitalization or death.

When researchers use the term efficacy, they are describing how a vaccine works under ideal, tightly controlled conditions such as clinical trials. Efficacy refers to how the vaccine works in the real world, when people lead normal lives. without the same controls in place.

Vaccines authorized in the United States have shown protection against symptomatic Covid-19 in clinical trials. The researchers investigated whether the vaccines prevented people from both tested positive for Covid-19 and showing at least one symptom. The studies did not measure whether the vaccines protect against asymptomatic infection, which simply means a positive test for Covid-19.

For example, in large, late-stage clinical trials conducted by vaccine manufacturers, the vaccine from Pfizer Inc. and its partner BioNTech SE last year was found to be 95% effective in preventing symptoms of Covid-19, while the Moderna Inc. vaccine was 94.1% effective. In the study of about 44,000 people on the Pfizer vaccine, 170 developed Covid-19 with at least one symptom. Of these, only eight had been vaccinated, while 162 had received a placebo. The 95% efficiency rate is derived from this report.


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