Just days after the release of key new guidelines on airborne coronavirus transmission, the Centers for Disease Control and Prevention withdrew the advice on Monday, saying it was “mistakenly posted” on the agency’s website.

The rapid reversal caused dismay among scientists and once again challenged the credibility of the world’s leading health agency despite President Trump and his senior health officials attempting to undermine CDC scientists.

The president faces an election the outcome of which could affect the public’s perception of his handling of the coronavirus pandemic.

The turning point came when the number of virus-related deaths in the United States neared the 200,000 mark. Tens of thousands of new infections are reported every day, and experts fear a resurgence as cooler weather approaches and people spend more time indoors.

The new document first recognized that the virus mainly spread by air. This is an urgent statement on how people should protect themselves indoors and how ventilation should be provided in schools, offices, hospitals and other public buildings.

Experts with knowledge of the incident said Monday that the recent reversal appeared to be more of a real flaw in the agency’s scientific review process than the result of political interference. Officials said the agency would publish revised guidelines soon.

“We are reviewing our process and tightening the criteria for reviewing all guidelines and updates before they are posted on the CDC website,” said Jason McDonald, an agency spokesman.

Nonetheless, the turnaround led to Rügen even among staunch CDC supporters. “It’s not something that gives you a lot of confidence, is it?” said Dr. Carlos del Rio, an infectious disease expert at Emory University. “It doesn’t help at all.”

Other scientists said it was difficult to understand how a document of such public health concern could have been published without careful scrutiny, given the scrutiny of the agency’s actions.

“At this point, everyone knows the stakes are extremely high when it comes to science communication,” said Dr. Abraar Karan, internist at Harvard Medical School.

The CDC has suffered a number of beatings as the pandemic spread across the United States. For example, officials didn’t recommend face covering for the public until April, after originally saying that masks were not necessary.

The CDC said in August that people who have close contact with an infected person but don’t have symptoms don’t need to be tested for the infection. But last week after the New York Times reported that the guidelines had been dictated by administration officials rather than scientists, the agency reversed its position, saying that all close contacts made by infected people should be tested regardless of symptoms.

This reversal came after top HHS spokesman Michael R. Caputo took a vacation to “focus on his health and the well-being of his family” after accusing federal scientists of “rioting” in a bizarre Facebook rant . Dr. Paul Alexander, an advisor to Mr. Caputo who was extremely critical of CDC research, is also leaving the department.

Mr Trump spoke out against the agency’s director, Dr. Robert Redfield, after Dr. Redfield had announced a hearing in Congress that a vaccine would not be generally available until the middle of next year. “It’s just false information,” said the president.

The constant controversy makes it “all the more difficult for the general public, who are now looking at this guide and wondering,” What the hell does this all mean? “Said Dr. Karan.

The most recent incident involves the airborne spread of the virus through droplets and aerosols, tiny particles that contain the virus that can remain in the air for long periods of time and move beyond three feet.

Scientists were aware from the beginning of the pandemic that the coronavirus can be transmitted through respiratory droplets that infected people sneeze or cough. It is only recently that health authorities such as the World Health Organization have recognized the role of swimming aerosols emitted through speaking, breathing, or even singing.

The CDC’s new document described both of them as airborne transmission, but officials had not previously described an expansive role for aerosols.

The virus spreads through “respiratory droplets or small particles, such as aerosols, which are produced when an infected person coughs, sneezes, sings, speaks or breathes,” says the document published on Friday and subsequently withdrawn.

These particles can be inhaled and cause infection, the agency added, “This is believed to be the main spread of the virus.”

“Airborne viruses, including Covid-19, are among the most contagious and easiest to spread,” the CDC also said – a statement with a huge impact on how hospitals should care for coronavirus patients, said Saskia Popescu, an epidemiologist at the George Mason University Hospital.

Viruses in the air may require that patients be isolated in what are known as negative pressure rooms, which prevent the virus from escaping, and that health care workers wear N95 masks at all times.

“The challenge would then be that we cannot place every single patient in negative pressure rooms,” said Dr. Popescu.

If the ventilation and infection control systems in hospitals offered inadequate protection against the virus, hospitals would have caused many more infections, she added.

“My gut tells me that’s why they really pulled it,” said Dr. Popescu. “I think they understand that you can’t just throw things away ‘in the air’ at random. This is having a very serious impact on hospitals. “

Scientific research to date shows that aerosols are especially important in certain environments – mainly in crowded indoor spaces with poor ventilation, such as many bars, clubs, gyms and restaurants.

In these rooms, the virus can hover in the air for long periods of time and travel distances of more than two meters, the agency warned in the document released on Friday.

Earlier this summer, scientists isolated live virus from aerosols collected seven and 16 feet away from an infected patient in a hospital. The spread in the air could explain many so-called “superspreader” events, according to scientists, for example a number of cases after a choir practice in Washington state and why the southern states saw an increase in infections this summer than people in air-conditioned rooms surroundings remained in the house.

The researchers noted on Sunday that the agency had updated its description of how the virus was transmitted to say the pathogen is mainly airborne. Many had welcomed the CDC’s recognition of these risks and the approval of indoor air filters.

“A lot of people spend hours cleaning seats and I think it’s pretty much an exaggeration, frankly,” said Dr. del Rio.

But the new language disappeared on Monday morning and the official council reverted to an earlier description of breath droplet spread. “A draft of the proposed changes to these recommendations was incorrectly posted on the agency’s official website,” the agency said.

The document was posted “ahead of schedule” on the CDC website and will be released after revision, according to a federal official familiar with the matter.

More than 200 experts studying aerosols appealed to the World Health Organization in July to review the evidence of aerosol transmission of the coronavirus.

The WHO acknowledged that this pathway appears to be a major contributor to the spread of the pandemic, but experts disagree on its importance in relation to the heavier breath droplets sneezed or coughed by infected patients.

“We really don’t have any epidemiological evidence right now that it is more than the other,” said Dr. Popescu.

Some experts said that whichever is more important – droplets or aerosols – it depends on how people are supposed to protect themselves.

“I think aerosols are very important, important enough for health counseling to be the focus,” said Linsey Marr, an airborne virus expert at Virginia Tech. “I hope it comes back in some form that recognizes the importance of aerosols.”