The delta variant of the coronavirus has been found in more than 80 countries since it was first detected in India and is responsible for 20% of infections in the U.S.
Most virus mutations aren't concerning, but there is a worry that some variants might evolve to be more contagious, cause more severe illness or evade the protection of vaccines.
The delta variant spreads more easily, but it's not clear yet whether it makes people sicker.
Fully vaccinated people appear protected from it, though the effectiveness of AstraZeneca and Pfizer's two-dose shots seems to drop for those with only one dose.