The spread of the Delta variant, an increase in vaccinated people testing positive for Covid-19, and the U.S. government’s call for booster shots have raised new questions about the power of the coronavirus vaccines. Here’s what we know about how authorized vaccines perform against the variant, and their ability to prevent infection and serious illness.

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

Health experts and scientists use a variety of terms and measures to describe how well a vaccine works. Vaccines can be evaluated based on their ability to prevent initial infection, symptomatic disease or severe illness that can lead to hospitalization or death.

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

The vaccines authorized in the U.S. were shown in clinical trials to protect against symptomatic Covid-19. Researchers looked at whether the vaccines prevented people from both testing positive for Covid-19 and showing at least one symptom. The studies didn’t measure if the vaccines protect against asymptomatic infection, which means simply testing positive for Covid-19.

For instance, in large late-stage clinical trials conducted by vaccine makers, the vaccine from Pfizer Inc . and partner BioNTech SE was found last year to be 95% effective at preventing symptomatic Covid-19, while the Moderna Inc . vaccine was 94.1% effective. In the approximately 44,000-person study of the Pfizer vaccine, 170 developed Covid-19 with at least one symptom. Of those, just eight had been vaccinated, while 162 had received a placebo. The 95% efficacy rate is derived from that ratio.