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Health officials in the United States will on Friday explain the science behind their u-turn on face masks, as Republicans express skepticism over the decision - which appears to have stemmed from research into a July 4 outbreak.

The Centers for Disease Control and Prevention (CDC) on Tuesday announced that they were updating their previous guidance to now recommend the wearing of face masks once more, when indoors and in parts of the country with substantial COVID-19 transmission.

They did not explain their reason for the shift in policy, and merely said it was due to new data on the highly contagious Delta variant. On May 13 the American public was told they no longer needed to wear masks indoors. 

An internal federal health document obtained by The Washington Post explained the new data, and reported that the Delta variant was as infectious as chickenpox or Ebola.

Officials, the document stated, must 'acknowledge the war has changed.' 

Dr Rochelle Walensky, the director of the CDC, has said that they will publish the science behind their decision, announced on Tuesday, on Friday. The CDC has faced some criticism for announcing new recommendations on face masks without providing the science behind their decision

The slide presentation said that the CDC must improve its messaging on COVID-19, and emphasize the urgency of the situation. 

'I finished reading it significantly more concerned than when I began,' said Robert Wachter, chairman of the Department of Medicine at the University of California at San Francisco. 

Dr Rochelle Walensky, director of the CDC, said that the new data - to be published on Friday - showed that vaccinated people infected with the Delta variant carry tremendous amounts of the virus in the nose and throat.

Walensky told The New York Times that the data suggest that even fully immunized people can be unwilling vectors for the virus - a change from the previously-held belief that vaccinated people were unlikely to increase the spread of COVID-19.

Walensky privately briefed members of Congress on Thursday, drawing on much of the material in the slide presentation obtained by The Washington Post. 

Walensky is pictured on July 20 testifying before Congress. She briefed Congress on the new scientific findings on Thursday, and on Friday will make the results public

'I think people need to understand that we're not crying wolf here. This is serious,' she told CNN.  

One of the slides states that there is a higher risk among older age groups for hospitalization and death relative to younger people, regardless of vaccination status. 

Another estimates that there are 35,000 symptomatic infections per week among 162 million vaccinated Americans.

The document outlines 'communication challenges' fueled by cases in vaccinated people, including concerns from local health departments about whether coronavirus vaccines remain effective and a 'public convinced vaccines no longer work/booster doses needed.'

The CDC was criticized this week for updating the mask guidance without detailing the science behind it.  

Kathleen Hall Jamieson, director of the Annenberg Public Policy Center at the University of Pennsylvania, told The Washington Post that their move violated scientific norms.

'You don't, when you're a public health official, want to be saying, 'Trust us, we know, we can't tell you how,' Jamieson said. 

'The scientific norm suggests that when you make a statement based on science, you show the science.

'And the second mistake is they do not appear to be candid about the extent to which breakthroughs are yielding hospitalizations.' 

Joe Biden has been strongly urging people to get vaccinated. On May 13 he celebrated the announcement that face masks were no longer necessary - something critics have seized upon

Kevin McCarthy, the most senior Republican in the House, claimed the House doctor told them the study was conducted in India using a vaccine that was not approved in the U.S.

'The mask mandate is based upon a study in India, based upon a vaccine that isn''t approved in America that didn't pass peer review. Could this be a plan to keep our schools closed?' he asked on Twitter.

The CDC cites research from India on viral loads as adding to global concerns about transmission post-vaccination, but they make clear that other research and additional studies were under way. 

McCarthy, however, tweeted a May 13 video of Joe Biden stating: 'Folks, if you're fully vaccinated — you no longer need to wear a mask.' 

McCarthy captioned the clip: 'Total hypocrisy.' 

Nancy Pelosi, the Speaker of the House, was asked about the backlash to the new mask mandates, and McCarthy's anger.

'He's such a moron,' she reportedly said, in a scarcely-audible clip. 

Yet at the same time, Democratic-run cities have said they cannot make changes without being in full possession of the facts.

Even officials in Democrat areas were unsure.

Mitchell Katz, president of New York City Health and Hospitals, said at a press conference with Mayor Bill de Blasio that he wanted more information.

'While the CDC issued their guidance yesterday at about 3 p.m., they have not yet released their scientific reports on the data that underlies their recommendation,' he said.

He added that his focus remained on getting people vaccinated.

'I think we owe it to New Yorkers to very carefully, as you say, review that information and understand its implications,' he said. 

The CDC experts have been paying particular attention to an outbreak in Provincetown, on Cape Cod, after the July 4 celebrations. 

As of Thursday, 882 people were tied to the Provincetown outbreak. 

The popular Cape Cod vacation resort of Provincetown is seen on July 24. The artistic and foodie city drew its usual large crowd for the July 4 weekend, with people believing that fully vaccinated people could not transmit the virus. It is now believed that that is not correct

Dressed as Maxine the Vaccine, Poppy Champlin encourages pedestrians to get vaccinated for COVID-19 while promoting her comedy show on Commercial Street in Provincetown, on July 24

Among those living in Massachusetts, 74 per cent of them were fully immunized, ABC News reported, yet officials said the vast majority were also reporting symptoms. 

Seven people were reported hospitalized.

The initial findings of the investigation led by the Massachusetts Department of Public Health, in conjunction with the CDC, seemed to have huge implications.

All indications now are that the Provincetown outbreak investigation is among the pieces of new evidence behind the CDC's decision to ask Americans to once again put on their masks indoors, even if they are vaccinated. 

The one glimmer of hope came from Britain, where the Delta variant has wreaked havoc, but is now dramatically slowing down.

Experts hope that this may indicate the U.S. surge could also be over soon. 

Last week, the leading British COVID modeler said that the country was 'almost certain' to hit 100,000 cases per day, and the U.K.'s daily case count crossed the 50,000 threshold for the first time since January.

Yet since July 20, cases have fallen fast. 

From a high of nearly 54,000 on July 17, the daily tally slid to 43,404 last Wednesday; and 28,652 on Sunday.

This Monday, the U.K.'s case count slipped below 25,000 - a 50 per cent reduction in a single week. 

'In the United Kingdom, cases are clearly coming down at this point,' said Dr Scott Gottlieb, the former Food and Drug Administration commissioner, on Monday. 

'If the U.K. is turning the corner, it's a pretty good indication that maybe we're further into this than we think, and maybe we're two or three weeks away from starting to see our own plateau here in the United States.'