What should I know about the delta variant? 

It's the most contagious coronavirus mutant so far in the pandemic, but COVID-19 vaccines still provide strong protection against it. Nearly all hospitalizations and deaths are among the unvaccinated.

Still, the U.S. Centers for Disease Control and Prevention cited the delta's surge for its updated advice that fully vaccinated people return to wearing masks indoors in areas with high transmission. The change is based on recent research suggesting that vaccinated people who get infected with the delta variant can spread it to others, even if the vaccinated don't get seriously ill. 

The new guidance helps protect the unvaccinated, including children who aren't yet eligible for the shots, and others who are at high-risk for serious illness if infected.

Some breakthrough cases with mild or no symptoms were always expected, since the vaccines were designed to prevent serious illness. The CDC no longer publicly counts those milder breakthrough cases, but a Kaiser Family Foundation analysis of data from states that keep a tally found they make up a tiny share of all COVID-19 infections.

It's not yet clear if the delta variant makes people sicker. But experts say it spreads more easily because of mutations that make it better at latching onto cells in our bodies. 

The delta, first detected in India, has quickly become dominant wherever it has landed, including the U.S. 

Viruses constantly mutate, and most changes aren't concerning. But the worry is that unchecked spread could fuel mutations and produce a variant that's even more contagious, causes more severe illness or evades the protection that vaccines provide.

It's why experts say making vaccines accessible globally is so critical. And they note the importance of being fully vaccinated; getting just one dose of the two-dose vaccines isn't as protective against the delta.