▶ Watch Video: Doctor discusses importance of COVID-19 vaccinations and wearing masks 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. “This finding is concerning and was a pivotal discovery leading to the CDC’s updated mask recommendation,” said Walensky. The scientists who authored the report, published Friday, said their finding amounted to a “crude correlation” to transmissibility, and cautioned that “microbiological studies are required to confirm these findings.” Beyond merely measuring the amount of virus someone is carrying, scientists would need to confirm the risk by culturing live virus from a specimen, a spokesperson for the Massachusetts Department of Public Health told CBS News. However, the investigation’s findings add to a growing body of evidence that has spurred federal health officials to ramp up their guidance to curb surging cases of the delta variant around the country. In Provincetown, CDC researchers — as well as local, state and university scientists and investigators — had turned up 469 cases of COVID-19 among Massachusetts residents who traveled to the Cape Cod resort starting on the Fourth of July weekend. A total of 346 of those cases, or around three in four patients, were in fully vaccinated people. A majority of those had symptoms and nearly all had the delta variant. However, no deaths were reported and only five were hospitalized, suggesting the vaccine’s goal of reducing the severity of the disease remained successful. An internal document within the agency recently urged health officials to step up their warnings around the virus, acknowledging that “the war has changed” in light of the new highly-contagious delta variant. Nationwide, with the delta variant now estimated to make up nearly all of the circulating virus in the United States, the average pace of new cases has climbed to rates not seen since mid-April. Walensky warned members of Congress on Thursday, according to a release from a House subcommittee, that hospitalizations had surged to levels beyond the peak of summer last year. The new data also comes as the public health agency had come under renewed criticism for not having released the data underlying its shifting guidance earlier this week, apart from vague references to the figures in press conferences and online posts. “This outbreak investigation is one of many CDC has been involved in across the country and data from those investigation will be rapidly shared with the public when available,” Walensky said.