(CNN) The US Centers for Disease Control and Prevention on Monday abruptly reverted to its previous guidance about how coronavirus is transmitted, removing language about airborne transmission it had posted just days earlier.
"A draft version of proposed changes to these recommendations was posted in error to the agency's official website. CDC is currently updating its recommendations regarding airborne transmission of SARS-CoV-2 (the virus that causes COVID-19). Once this process has been completed, the update language will be posted," Jason McDonald, a CDC spokesman, said in a response emailed to CNN.
The guidance pertained to the way the novel coronavirus is spread. While it's known it can spread through droplets among people standing less than 6 feet apart, research has continued to explore how the virus suspends in aerosolized particles in the air and transmitted to people more than 6 feet away.
The CDC transmission guidance acknowledging airborne transmission had been quietly posted on Friday, according to the agency's website. CNN was first to report the change on Sunday. The CDC responded to CNN just before noon on Monday to say it was reverting to the previous guidance.
A federal official familiar with the situation said there was no political pressure involved in the change.
"This was totally the CDC's doing," the official said. "It was posted by mistake. It wasn't ready to be posted."
The official said the guideline change was published without being thoroughly reviewed by CDC experts.
"Somebody hit the button and shouldn't have," the official said.
The agency tried to further clarify what it meant by aerosol transmission, the official said. "It can occur, but it's not the way the virus is primarily being transmitted," the official said. But in the effort to say that, it was written in such a way "that it's being understood to mean it's more transmissible than we thought, which is not the case."
The official added that the guidance is "getting revised," but didn't say when the revision would be posted to the CDC's website.
Despite several studies that have shown the novel coronavirus can spread through small particles in the air, the CDC page now says that Covid-19 is thought to spread mainly between people in close contact -- about 6 feet -- and "through respiratory droplets produced when an infected person coughs, sneezes or talks." This is the same language the agency posted months ago.
In language posted Friday and now removed, CDC said Covid-19 most commonly spread between people who are in close contact with one another, and went on to say it's known to spread "through respiratory droplets or small particles, such as those in aerosols, produced when an infected person coughs, sneezes, sings, talks or breathes."
These particles can cause infection when "inhaled into the nose, mouth, airways, and lungs," the agency said. "This is thought to be the main way the virus spreads."
"There is growing evidence that droplets and airborne particles can remain suspended in the air and be breathed in by others, and travel distances beyond 6 feet (for example, during choir practice, in restaurants, or in fitness classes)," the page said in the Friday update, which has since been removed. "In general, indoor environments without good ventilation increase this risk."
In the Friday update, the CDC had added new measures to protect yourself in others, including recommendations to use air purifiers to reduce airborne germs in indoors spaces and clear guidance to "stay at least 6 feet away from others, whenever possible." The updated CDC page had also changed language around asymptomatic transmission, shifting from saying "some people without symptoms may be able to spread the virus" to saying "people who are infected but do not show symptoms can spread the virus to others." That language has now been removed.
Many researchers and doctors have said for months that coronavirus can be transmitted through small airborne viral particles. In July, 239 scientists published a letter that urged the World Health Organization and other public health organizations to be more forthcoming about the likelihood that people could catch the virus from droplets that were floating in the air.
Donald Milton, an author of the letter and a professor of environmental health at the University of Maryland, said he had been "encouraged" when he saw the revised CDC guidance over the weekend, but he said he suspected it was a work in progress since the rest of the CDC's site wasn't updated to reflect the changes.
"I think that the science behind what turned out to be a draft statement is strong and agrees with my understanding of the data," Milton said. "I'm very happy to know that CDC is working on incorporating the latest science in its public statements about transmission. Today, we know a lot about aerosols and how to control them to prevent transmission."
Dr. Mike Ryan, the executive director of World Health Organization Health Emergencies Programme, said that while the United States waits on final advice from the CDC, the world cannot wait to find ways to stop the spread of the disease.
"Based on the evidence, [WHO] believes there is a wide range of transmission modes," Ryan said. "We believe the disease is predominately or primarily spread through droplets spread and through larger droplet nuclei. But we have always said that smaller droplet nuclei can spread this disease -- and that is very context driven."
Ryan explained that people who are in a small indoor area with poor ventilation can become infected through aerosol-based transmission. It's all about knowing risk and "managing the frequency, intensity and duration" of time spent around others in crowded spaces, he said.
"We've got to become able to accept that there are very few absolutes in this response," Ryan added. "We've got to be able to be smart, and make smart decisions, the smart decisions are made based on understanding risk, minimizing risk, and then being aware of the residual risk, and as best we can to avoid that."
Some were concerned that the rapid updates from CDC might be linked to earlier reports of political pressure and interference at the agency.
On Friday, CDC updated its coronavirus testing guidance to stress that anyone who has been in contact with an infected person should be tested for coronavirus. An earlier, controversial update was not written by CDC scientists and was posted online before it had undergone the normal scientific review process, two sources confirmed to CNN last week.
CNN also reported last week that US Health and Human Services communications officials had recently pushed to change language of weekly science reports released by the CDC so as not to undermine President Donald Trump's political message, according a federal health official told CNN. CDC Director Dr. Robert Redfield said last week "at no time has the scientific integrity" of these reports been compromised.
On Monday, Dr. Leana Wen, an emergency physician at George Washington University and CNN medical analyst, said she worried that the sudden changes to CDC guidance could be motivated by politics, rather than science.
"The fact that they retracted this, even though this is common scientific knowledge at this point, one has to wonder what's behind it," she said. "Was there political pressure? Political interference that's driving this rather than science?"
Science, she said, should always trump politics at the CDC.
"It's not about politics," Wen said. "It's not about what looks good. It's about following the science so that we can reduce the number of infections and reduce deaths."
Regardless, the CDC's reversal could confuse people -- and hamper mitigation efforts that could be put in place.
"It's extremely confusing, and that type of whiplash -- especially without an explanation directly from the CDC -- creates confusion and unfortunately leads to lack of trust in the CDC overall," Wen said.