The US Centers for Disease Control and Prevention has again updated its guidance about how Covid-19 spreads to include information about potential for airborne transmission.
"CDC continues to believe, based on current science, that people are more likely to become infected the longer and closer they are to a person with COVID-19," the agency said in a statement on Monday.
"Today's update acknowledges the existence of some published reports showing limited, uncommon circumstances where people with COVID-19 infected others who were more than 6 feet away or shortly after the COVID-19-positive person left an area. In these instances, transmission occurred in poorly ventilated and enclosed spaces that often involved activities that caused heavier breathing, like singing or exercise. Such environments and activities may contribute to the buildup of virus-carrying particles."
Still, the available data suggest "it is much more common for the virus that causes COVID-19 to spread through close contact with a person who has COVID-19 than through airborne transmission," the new guidance says.
How Covid-19 spreads
The agency still says Covid-19 is thought to mainly spread through close contact between people -- within 6 feet of each other.
"When people with COVID-19 cough, sneeze, sing, talk, or breathe they produce respiratory droplets. These droplets can range in size from larger droplets (some of which are visible) to smaller droplets," the guidance says. "Small droplets can also form particles when they dry very quickly in the airstream."
But the CDC now says the virus can spread through small particles of virus that linger in the air and may be able to infect people more than 6 feet away. In enclosed spaces with poor ventilation, a person who was breathing heavily -- while singing or exercising, for example -- can infect people who aren't nearby, even after the person with Covid-19 has left the space.
The new guidance is also clear about asymptomatic transmission: "People who are infected but do not show symptoms can also spread the virus to others," it says.
However, CDC says Covid-19 spreads "less commonly" through contact with contaminated surfaces, and "rarely" between people and animals.
CDC says people can protect themselves from the coronavirus by staying 6 feet away from others, wearing a mask that covers the nose and mouth, washing hands frequently, cleaning surfaces and staying home when sick.
CDC says previous guidance was posted in error
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 floating in the air.
Some said they were encouraged last month when the agency updated its guidance to say Covid-19 could spread through the air.
But days later, CDC abruptly reverted to its earlier guidance, which did not include information about airborne transmission. The agency said a draft version of proposed changes had been posted in error, and the guidance would be updated again once the scientific review process was completed.
The unvetted post followed reports of interference in other CDC guidance and reports and sparked concerns about political pressure.
"The fact that they retracted this, even though this is common scientific knowledge at this point, one has to wonder what's behind it," said Dr. Leana Wen, an emergency physician at George Washington University and CNN medical analyst, said last month. "Was there political pressure? Political interference that's driving this rather than science?"
In a statement on Monday, CDC said its recommendations to prevent spread remained the same "based on existing science and after a thorough technical review of the guidance."
"The best way to prevent illness is to avoid being exposed to this virus," it says.