The Centers for Disease Control and Prevention issued updated guidance on Tuesday recommending that vaccinated people wear masks in indoor, public settings if they are in parts of the U.S. with substantial to high transmission, among other circumstances.

Why it matters: The guidance, a reversal from recommendations made two months ago, comes as the Delta variant continues to drive up case rates across the country. Millions of people in the U.S. — either by choice or who are ineligible — remain unvaccinated and at risk of serious infection.

Details: Community leaders in areas with high transmission rates should encourage vaccination and masking, the agency says.

• In another reversal, the CDC also recommends universal indoor masking for all teachers, staff, students and visitors to K-12 schools this incoming school year, regardless of vaccination status.
• Los Angeles County, New Orleans, Savannah and Chicago are among the major metropolitan areas that reinstated mask mandates amid a rise in cases.

What's happening: Senior officials were in talks this past weekend over whether to alter masking guidance after emerging CDC data showed that vaccinated individuals infected with the Delta variant have viral loads similar to those who are unvaccinated.

• Virtually all hospitalizations and deaths continue to be among the unvaccinated, especially in places with low vaccination rates.
• In rare occasions, CDC director Rochelle Walensky said some vaccinated people can contract the Delta variant in a breakthrough infection and "may be contagious."
• Nearly 50% of the U.S. population 12 years and older is fully vaccinated.