CDC says "pivotal discovery" about delta variant prompted new mask guidance and urges universal masking in some places

The Centers for Disease Control and Prevention is urging local health authorities to consider heightened prevention strategies like "universal masking" for public indoor settings, particularly when it comes to large indoor gatherings with travelers. It cited new data from a delta variant outbreak in Massachusetts that led to hundreds of coronavirus infections — many among fully vaccinated travelers.

The CDC, in the agency's Morbidity and Mortality Weekly Report, also published more details about the "pivotal discovery" that prompted federal health officials to ramp up their masking guidance earlier this week. Tests used to diagnose cases linked to the Massachusetts outbreak had similar "cycle threshold" values among both unvaccinated and fully vaccinated "breakthrough" cases, suggesting both groups could be carrying similar loads of the virus. "High viral loads suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people infected with Delta can transmit the virus," CDC Director Rochelle Walensky said in a statement.