Covid-19 can spread through tiny particles that can linger in the air for hours, according to new guidance from the Centres for Disease Control and Prevention.
The CDC has until now not made a direct link about the role airborne transmission is playing in the spread of the virus.
But the US public health agency's new advice admits aerosolised virus particles can, and do, infect others under "certain circumstances".
The capacity of Covid-19 to spread through minuscule droplets, which float in the air long after an infected person coughs, breathes, or sneezes, was noted by authorities in China in February, very early in the outbreak.
Dozens of studies conducted worldwide have since confirmed or suggested it, with growing evidence pointing to aerosol transmission.
Last month the CDC posted an update saying the virus could spread through aerosols, before hastily removing it, saying the advice was included in an “early draft” that had been published in error.
The new guidance acknowledges the virus can linger in the air – though it insists aerosolised transmission remains “uncommon”.
“There is evidence that under certain conditions, people with Covid-19 seem to have infected others who were more than six feet away,” says the guidance, which was issued on Monday.
“These transmission events appear uncommon and have typically involved the presence of an infectious person producing respiratory droplets for an extended time (greater than 30 minutes to multiple hours) in an enclosed space.
“Enough virus was present in the space to cause infections in people who were more than 6 feet away or who passed through that space soon after the infectious person had left.”
Circumstances where confirmed airborne transmission has occurred, according to the CDC, include in enclosed spaces, when people were exposed shortly after the infected person left.
Aerosolised transmission has also occurred when infected people were shouting, singing or exercising, which increased the concentration of “suspended respiratory droplets in the air space”.
In addition, inadequate ventilation can increase the risk of airborne transmission by allowing the build-up of “suspended small respiratory droplets and particles,” says the CDC.
The agency said most people become infected by inhaling large droplets through close contact with sufferers.
However, writing in the journal Science this week, a group of researchers suggested incidents involving aerosol transmission were more common than the CDC has acknowledged.
“Individuals with Covid-19, many of whom have no symptoms, release thousands of virus-laden aerosols and far fewer droplets when breathing and talking,” they said.
“Thus, one is far more likely to inhale aerosols than be sprayed by a droplet, and so the balance of attention must be shifted to protecting against airborne transmission.”
Cases in which the coronavirus spread through the air include a bus trip in China where 23 out of 68 passengers became infected with the virus, some of whom were sitting farther than the 6ft experts say large droplets can travel.
In another instance, a singer at a choir practice in the US spread the virus to 52 people, including one person who was sitting 45 feet away.
Mounting evidence prompted a group of more than 200 aerosol biologists and other experts to sign a letter addressed to the World Health Organisation in July calling for greater recognition of the role of aerosol transmission in the outbreak.
"It is becoming clear that the pandemic is driven by superspreading events, and that the best explanation for many of those events is aerosol transmission," Jose-Luis Jimenez, a researcher at the University of Colorado Boulder, and one of 239 scientists to sign the letter, told The Washington Post.
