In May, when C.D.C. officials lifted masking recommendations, they cited research showing that fully vaccinated people were unlikely to become infected with the virus, even with asymptomatic infections. But the variant’s talent for even partial immune evasion makes researchers nervous, as it suggests that fully vaccinated people may sometimes pick up asymptomatic infections and unknowingly spread the virus to others even if they never become ill. The Delta variant can infect vaccinated people, although its ability to do so is very limited, said Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health. “If you are in a place where cases are climbing, wearing a mask indoors in crowded public spaces is a way to keep yourself from contributing to the spread of Delta,” he said. Other scientists stop short of recommending that fully vaccinated people always wear masks indoors, but some now suggest that this may be appropriate depending on local circumstances — for example, wherever the virus is circulating in high numbers, or vaccination rates are very low.