Cities and states say they want to see data that led CDC to make mask U-turn ...

The Centers for Disease Control and Prevention is coming under increasing pressure to release the data that prompted it to reverse its mask guidelines on Tuesday, as cities and states say they need to see the evidence before revising their own policies. 

Federal officials say unpublished data showed vaccinated people infected with COVID-19 may be able to transmit the virus, leading them to recommend that everyone should wear a mask indoors in areas of high transmission. 

Republicans have poured doubt on the new findings and even Democratic-led cities say they won't make changes without understanding the facts. 

Iowa Governor Kim Reynolds said the change was confusing.

'I haven't seen the data - they're not really forthcoming on a lot of data that they're using to make this change in the guidance,' she said at a press conference. 

'And I really haven't seen any data that would support that states that don't issue a mask mandate versus states that do issue a mask mandate - that the statistics are that much different.'

Scientists have also asked for the data to be released.

'They're making a claim that people with delta who are vaccinated and unvaccinated have similar levels of viral load, but nobody knows what that means,' Dr Gregg Gonsalves, an associate professor at the Yale School of Public Health, told The Washington Post. 

When CDC Director Dr. Rochelle Walensky announced the reversed policy on Tuesday she said 'new scientific data' justified the move. She said it came from research on 'breakthrough' infections, comparing viral loads between vaccinated and unvaccinated people.

But the CDC has yet to offer more specific information about the studies.

That information vacuum has been filled by the likes of House Minority Leader Kevin McCarthy claiming a doctor told him the study was conducted in India using a vaccine that was not approved in the U.S. 

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

At a press conference with Mayor Bill de Blasio, New York City Health and Hospitals President Mitchell Katz said: '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 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. 

A day earlier, the country's top infectious diseases expert Anthony Fauci told MSNBC that the latest information suggested that even vaccinated people can spread Delta and explains 

Previous research indicated that people infected with Delta have 1,000 times more copies of the coronavirus in their respiratory tracts compared to those infected with older strains of the virus. 

Since the start of the pandemic, one common metric used by scientists is 'viral load' - a measurement of how many copies of the coronavirus are in a

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