Evidence that Covid-19 may be airborne

"...The possibility of airborne transmission in public settings - especially in very specific conditions, crowded, closed, poorly ventilated settings that have been described, cannot be ruled out," said Benedetta Allegranzi, the WHO's technical lead for infection prevention and control.

"We have been talking about the possibility of airborne transmission and aerosol transmission as one of the modes of transmission of Covid-19," she said.

"However, the evidence needs to be gathered and interpreted, and we continue to support this."

Allegranzi was responding to a question about an open letter from more than 200 scientists had accused the organisation of underestimating the possibility of airborne transmission. The WHO has so far said that the virus is transmitted through droplets when people cough or sneeze.

Open letter to the WHO

"We wanted them to acknowledge the evidence. This is definitely not an attack on the WHO. It's a scientific debate, but we felt we needed to go public because they were refusing to hear the evidence after many conversations with them," said Jose Jimenez, a chemist at the University of Colorado one of the signatories of the letter.

"In healthcare settings, if aerosol transmission poses a risk then we understand healthcare workers should really be wearing the best possible preventive equipment... and actually the World Health Organisation said that one of the reasons they were not keen to talk about aerosol transmission of Covid-19 is because there's not a sufficient number of these kind of specialised masks for many parts of the world," he told Reuters.

"And in the community, if we're thinking about aerosol transmission being a particular risk, then we need to think about how to prevent larger super spreading events, larger outbreaks and those occur in indoor environments with poor ventilation, with crowding and with prolonged close contact."

WHO officials have cautioned the evidence is preliminary and requires further assessment.