It’s when a fully vaccinated person gets infected with the coronavirus.

In studies, the two-dose COVID-19 vaccines by Pfizer and Moderna were around 95% effective at preventing illness, while the one-shot Johnson & Johnson shot was 72% effective, though direct comparisons are difficult. So while the vaccines are very good at protecting us from the virus, it’s still possible to get infected with mild or no symptoms, or even to get very sick.

If you do end up getting sick despite vaccination, experts say the shots help reduce the severity of the illness — the main reason to get vaccinated.

The understanding of how vaccinated people who are infected might spread the virus to others is changing.

Previously, health officials believed vaccinated people who get breakthrough infections were unlikely to spread the virus. But with the more contagious delta variant that is now dominant, the U.S. Centers for Disease Control and Prevention said new data shows people who get infected could carry enough virus in their noses and throats to spread it to others.

The agency recently cited the data in updating its guidance to say vaccinated people should go back to wearing masks indoors in areas where the virus is surging.

“It is concerning enough that we feel like we have to act,” said CDC Director Dr. Rochelle Walensky.

Still, health experts say the vaccines provide strong protection against serious illness. In the U.S., people who weren’t vaccinated make up nearly all hospitalizations and deaths from COVID-19.