On Thursday 9 April, the World Health Organization (WHO) acknowledged for the first time that the coronavirus might be airborne with the potential to be transmitted several metres in the air across poorly-ventilated rooms.
For months the WHO has denied the importance of aerosols, sustaining that the virus is spread mainly by contaminated surfaces and larger droplets produced by coughing, sneezing, or talking. These bigger droplets fall to the ground much more quickly – within about 2 metres – because they are too heavy to ride on light air currents. Whereas, lighter aerosols – tiny droplets of less than 5 micrometres in diameter – can accumulate in the air and travel much longer distances.
Governments are starting to change policies on their own to combat airborne transmission in light of growing evidence that aerosols can carry SARS-CoV-2, the coronavirus that causes the COVID-19 infection.
Many have criticised the WHO for being too slow to update guidelines. For example, the organisation did not change its stance on face masks until early April, despite calls from clinicians and researchers, and even though many governments had already mandated their use.
In a commentary in the journal Clinical Infectious Diseases, a group of more than 200 clinicians, infectious-disease physicians, epidemiologists, engineers, and aerosol scientists urged the medical community and public health authorities to recognise the potential for airborne transmission.
“We are concerned that the lack of recognition of the risk of airborne transmission of COVID-19 and the lack of clear recommendations on the control measures against the airborne virus will have significant consequences: people may think that they are fully protected by adhering to the current recommendations, but in fact, additional airborne interventions are needed for further reduction of infection risk”, they wrote.
The WHO guidelines follow the dominant view dating back to the 1930s and 1940s that respiratory viruses are transmitted by the larger droplets or through contact with droplets that fall on surfaces or are transferred by people’s hands. However, recent experiments suggest a more complex story, with aerosols as an important transmission route.
At a press conference on 7 July, the WHO finally announced that it will issue new guidelines about transmission in poorly ventilated settings with close contact. Although the WHO has softened its stance on airborne transmission, the organisation maintains that current evidence still falls short.
Benedetta Allegranzi, technical leader of the WHO task force on infection control said: “We have to be open to this evidence and understand its implications regarding the modes of transmission, and also regarding the precautions that need to be taken”.
Allegranzi says that while the WHO acknowledges the possibility of aerosol transmission, there is still not enough evidence to make the case. But the WHO is evaluating emerging evidence with aerobiologists and engineers to develop better ventilation guidelines.
She added that the recommendation of frequent hand washing, physical distancing, quarantine, and mask-wearing will offer some protection if airborne transmission of COVID-19 is indeed occurring.
(1) Morawska, L. and Milton, D.K. It is Time to Address Airborne Transmission of COVID-19. Clinical Infectious Diseases (2020). DOI: 10.1093/cid/ciaa939