With concern over the COVID-19 Delta variant, the CDC has changed its mask recommendations. On Tuesday, the Centers for Disease Control and Prevention announced that people in areas with “high” or “substantial” COVID-19 transmission should resume wearing masks indoors, CNN reports.

According to CDC data, nearly two-thirds of America’s counties have high or substantial transmission of COVID-19. These counties are spread throughout the country, but appear to be concentrated in the southeast, west coast, and mountain regions. Almost 50 percent of US counties have a “high” level of community transmission.

The CDC initially revised its mask guidance in May, stating that people who are fully vaccinated didn’t need to wear masks indoors. At the time of the May guidance, CDC director Rochelle Walensky expressed a common feeling: “We have all longed for this moment when we can get back to some sense of normalcy.”

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However, that sense of normalcy will have to be delayed. Scientific data has shown that the “Delta variant behaves uniquely differently from past strains of the virus that cause COVID-19,” Walensky told reporters Tuesday.

This uniqueness allows the Delta variant to be “more transmissible” than other variants, a senior administration official said.

While officials believe the overwhelming majority of spread is still from unvaccinated people, emerging data suggests some vaccinated people can spread it, which is the driving factor behind Tuesday’s decision, CNN reports.