Under growing pressure from researchers, the World Health Organization acknowledged Thursday that the coronavirus can linger in the air indoors and potentially infect people even when they practice social distancing.
The United Nations agency had maintained that such airborne transmission occurred only during certain medical procedures and that nearly all infections occur when people inhale respiratory droplets expelled in their immediate vicinity or when they touch contaminated surfaces.
But mounting evidence — including “super-spreading” events in which multiple choir singers, restaurant diners or dance students were infected — suggests that the virus can be transmitted through microscopic droplets known as aerosols that can float in the air, potentially for hours.
This week, 239 researchers wrote an open letter to the WHO, urging officials to accept the possibility that aerosols were an important contributor to the spread of the virus.
In revised guidelines issued Thursday, the WHO recommended avoiding enclosed spaces with poor ventilation as well as crowded places. But it did not substantially change its position on masks, maintaining that they need to be worn only when social distancing of at least six feet is not possible.
The organization said that airborne transmission had not been definitively demonstrated but granted that it was a possibility in outbreaks such as one that sickened 53 of 61 choir members who attended a March 10 practice in Washington state. Two of the singers died.
“In these events, short-range aerosol transmission, particularly in specific indoor locations, such as crowded and inadequately ventilated spaces over a prolonged period of time with infected persons cannot be ruled out,” the new guidelines say.
The WHO said that more research is “urgently needed to investigate such instances and assess their significance for transmission of COVID-19.”
Scientists who signed the letter said that the new guidelines did not go far enough in heeding the evidence of airborne spread.