View Comments The Centers for Disease Control and Prevention announced in May that fully vaccinated individuals didn't need to wear masks in most settings — but that guidance changed Tuesday in light of new data and rising cases. Now even fully vaccinated Americans should wear masks indoors in areas of high COVID-19 transmission, according to the new guidance. Although the majority of new infections in the U.S. are in unvaccinated people, “breakthrough” infections, which generally cause milder illness, can occur in vaccinated people. The new guidance was in part prompted by data showing that the highly-transmissible delta variant can be spread by vaccinated people, according to the CDC.