The delta variant is more contagious than its predecessors, but COVID-19 vaccines still provide strong protection against it.
The U.S. Centers for Disease Control and Prevention cited the variant's surge in advising that vaccinated people return to wearing masks indoors in areas with high transmission.
The change is based on new research suggesting vaccinated people who get infected can spread it to others, even if the vaccinated don't get seriously ill.
It helps protect the unvaccinated, including children not yet eligible for the shots.
Some breakthrough cases were always expected. A Kaiser Family Foundation analysis found such cases remain rare.