On Monday morning the Centers for Disease Control and Prevention swiftly edited its web page describing how the novel coronavirus spreads, removing recently added language saying it was "possible" that the virus spread via airborne transmission. This was the third major revision to CDC information or guidelines published since May.

The agency had posted information Friday stating the virus can transmit over a distance beyond 6 feet, suggesting that indoor ventilation is key to protecting against a virus that has now killed nearly 200,000 Americans.

The CDC shifted its guidelines on Friday, but the change was not widely noticed until a CNN report on Sunday. Where the agency previously warned that the virus mostly spreads through large drops encountered at close range, on Friday it had said "small particles, such as those in aerosols" were a common vector.

But Jay Butler, the CDC's deputy director for infectious disease, said the Friday update was posted in error. "Unfortunately an early draft of a revision went up without any technical review," he said.

The edited web page has removed all references to airborne spread, except for a disclaimer that recommendations based on this mode of transmission are under review. "We are returning to the earlier version and revisiting that process," Butler said. "It was a failure of process at CDC."

For months, scientists and public health experts have warned of mounting evidence that the novel coronavirus is airborne, transmitted through tiny droplets called aerosols that linger in the air much longer than the larger globs that come from coughing or sneezing.

Experts who reviewed CDC's Friday post had said the language change had the power to shift policy and public behavior. Some suggested it should drive a major rethink of public policy — particularly at a time when students in many areas are returning to indoor classrooms.

It was a "major change," Jose-Luis Jimenez, a chemistry professor at the University of Colorado at Boulder who studies how aerosols spread the virus, told the Washington Post before the CDC reversed itself. "This is a good thing, if we can reduce transmission because more people understand how it is spreading and know what to do to stop it."

Since the pandemic began, experts have debated the ways the virus travels - and the methods to best halt it. At first, widespread fear of contaminated surfaces led some to bleach their groceries and mail. But the CDC soon concluded that person-to-person transmission was a much more pressing threat. Instead, the agency focused its guidance on avoiding the larger droplets hacked up by sneezes and coughs, which are thought to be mostly limited to a six-foot radius.

"We have been saying 'wear a mask' and '6 feet apart' for months," tweeted Abraar Karan, a physician at Brigham and Women's Hospital in Boston and Harvard Medical School. Karan said six feet of separation may be insufficient particularly in poorly ventilated indoor areas.

Some researchers suspected that the virus could travel much farther, especially indoors and in places where people talk loudly or sing. Infamously, one infected person in March unknowingly passed the coronavirus to 52 others at a choir practice in Washington state. Similar indoor "superspreader" events added weight to the idea of an airborne threat.

The World Health Organization recognized the threat of aerosols in July, after hundreds of scientists urged the international body to address airborne spread. It's not clear why the CDC finally followed; Jimenez said high-ranking CDC officials were still arguing publicly against airborne transmission as a major vector as recently as late August.

"Evidence has been accumulating for some time. Those of us who have been studying this were frustrated that the change was slow, but it finally came," Jimenez said.

When asked who wrote the draft that was posted in error, Butler said "I don't have all of that information. Obviously I'm asking some of the same questions."

The CDC is "very intensively" discussing guardrails in the publication process to prevent a repeat error. "This cannot happen again," Butler said. In May, the CDC updated an information page that suggested the coronavirus did not spread easily from contaminated surfaces. It also edited that revision after the update received widespread media attention to clarify that tweak was "not a result of any new science."

And last week, the CDC reversed testing guidelines to once again recommend that anyone, regardless of symptoms, who has been in close contact with an infected person be tested. The White House coronavirus task force had directed the agency to change those guidelines in August, allowing that asymptomatic people did not need to be tested.