The World Health Organization on Thursday formally acknowledged that droplets carrying the coronavirus may be airborne indoors and that people who spend long periods in crowded settings with inadequate ventilation may be at risk of becoming infected, a reversal that many scientists said was long overdue.
The agency also acknowledged unequivocally that the virus can be transmitted by people who do not have symptoms.
The admission came after a push by more than 200 experts prompted the agency to update its description of how the virus is spread. The agency now says transmission of the virus by aerosols, or tiny droplets, may have been responsible for “outbreaks of Covid-19 reported in some closed settings, such as restaurants, nightclubs, places of worship or places of work where people may be shouting, talking, or singing.”
The W.H.O. still largely emphasizes the spread of the virus by larger droplets that are coughed or inhaled, or from contact with a contaminated surface, also known as “fomite transmission.” And in a longer document on the scientific evidence, the agency still maintains that “detailed investigations of these clusters suggest that droplet and fomite transmission could also explain human-to-human transmission within these clusters.”
In addition to avoiding close contact with infected people and washing hands, people should “avoid crowded places, close-contact settings, and confined and enclosed spaces with poor ventilation,” the W.H.O. has said. It said homes and offices should ensure good ventilation.
“It is refreshing to see that W.H.O. is now acknowledging that airborne transmission may occur, although it is clear that the evidence must clear a higher bar for this route compared to others,” said Linsey Marr, an aerosol expert at Virginia Tech.
Still, the updated guidance is not as extensive as many experts hoped to see.
The W.H.O. had previously maintained that airborne spread is a concern only when health care workers are engaged in certain medical procedures that produce aerosols. But mounting evidence has suggested that in crowded indoor spaces, the virus can stay aloft in the air for hours and infect others when inhaled, and may even seed super-spreader events.
It has been widely accepted for months that seemingly healthy people can spread the virus. But from the beginning of the pandemic, the W.H.O. has maintained that asymptomatic cases were infrequent, and that asymptomatic transmission, while it may occur, was “very rare.”
On Thursday, however, the agency said: “Infected people can transmit the virus both when they have symptoms and when they don’t have symptoms.”
The statement provides an explicit rationale for everyone to wear masks — the W.H.O. endorsed them only in early June, long after most national governments did — and for more widespread testing even of people without apparent symptoms.