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Vaccinated people should be tested for COVID-19 and mask up after coming into contact with an infected person, according to released this week.

The change quietly accompanied the agency's new masking recommendations for areas of substantial and high transmission and K-12 schools, as data continue to accrue about the more transmissible Delta variant and the risk for vaccinated people contracting and spreading the virus.

The CDC now says even if someone is fully vaccinated, if they have a known exposure to "suspected or confirmed" COVID-19, they should be tested 3 to 5 days afterward. Fully vaccinated people should then wear a mask in indoor settings for 14 days or until they test negative for the virus, the agency added.

"Our updated guidance recommends vaccinated people get tested upon exposure regardless of symptoms. Testing is widely available," CDC Director Rochelle Walensky, MD, by email.

Testing vaccinated people took on new significance, given the CDC's Tuesday press briefing where the agency teased new data about the Delta variant. They explained that not only can vaccinated people transmit the virus, but they have comparable viral loads to unvaccinated people. late Thursday referenced a report where 882 COVID-19 cases were linked to a Fourth of July celebration in Provincetown, Massachusetts, and three-quarters of those were fully vaccinated.

CDC documents elaborated about how viral loads were nearly identical between 80 vaccinated people and 65 unvaccinated people in that outbreak (median Ct values 21.9 and 21.5, respectively).

The documents also noted that breakthrough cases reported to national surveillance have around a 10-fold increase in viral load for the Delta variant compared with Alpha and other strains.

"Vaccines prevent >90% of severe disease, but may be less effective at preventing infection or transmission -- therefore, more breakthrough and more community spread despite vaccination," according to the CDC's internal document.

The agency shared data showing the Delta variant is as transmissible as chicken pox, and more transmissible than Ebola, the 1918 flu, and smallpox. Not only that, but data from Canada, Singapore, and Scotland suggested Delta might also be more virulent than other strains, with higher odds of hospitalization, ICU admission, or death.

In terms of next steps for communicating these findings, CDC must "acknowledge the war has changed," the document stated.

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