KENTUCKY — In another dispiriting setback for the nation’s efforts to stamp out the coronavirus, scientists who studied a big COVID-19 outbreak in Massachusetts concluded that vaccinated people who got breakthrough infections carried about the same amount of the coronavirus as those who did not get the shots.

The study was cited in a new report from the Centers for Disease Control and Prevention (CDC) Friday, further solidifying what health experts had feared – vaccinated people can still transmit the delta variant.

This discovery prompted the CDC to recommend mask wearing again in areas where COVID-19 transmission is substantial or high. An internal memo from the CDC said it in no uncertain terms: the discovery makes it clear that officials must "acknowledge the war has changed" against COVID-19.

Leaked internal documents on breakthrough infections and the delta variant suggest the CDC may be considering other changes in advice on how the nation fights the coronavirus, such as recommending masks for everyone and requiring vaccines for doctors and other health workers.

The delta variant, first detected in India, causes infections that are more contagious than the common cold, flu, smallpox and the Ebola virus, and it is as infectious as chickenpox, according to the documents, which mentioned the Massachusetts cases.

The pivot in guidance wasn't welcome by all. Northern Kentucky Congressman Thomas Massie (R) still opposes masking and was frustrated to see masking guidance return, arguing that it could somehow increase vaccine hesitancy.

"I think the advisement of the CDC to those who have been vaccinated, that they need to put their masks back on, is actually going to cause more vaccine hesitancy," Massie said Friday. "I mean, people were promised, you get this vaccine, you get your freedom back."

Louisville Congressman John Yarmuth (D) acknowledged frustration with the CDC regarding clarity, but also said he's concerned for young people who can't yet be vaccinated.

"I don’t think there’s any question the CDC has been less than helpful – less than maximally helpful – in explaining what the situation is," Yarmuth said. "There’s a great deal of cause for concern, particularly with school."

What hasn’t changed in this fight, doctors say, is that COVID-19 vaccines are the best defense against getting severely ill.

"Luckily, the vaccines still provide us a very good efficiency against the delta variant; in particular, in making sure that you’re not hospitalized," said Jason Smith, chief medical officer for UofL Health.

Walensky made an appeal to unity on Friday, stressing that the situation could be turned around in a matter of weeks.

"If we unify as a country – people are getting vaccinated, putting their masks back on in those areas of high transmission – it is possible to turn this around in just a matter of weeks, and that’s really what I’m hoping we can do," she said.

The Associated Press contributed to this report.