The World Health Organization is examining evidence around the airborne transmission of SARS-COv2, the organisation has said. “We acknowledge that there is emerging evidence [on airborne transmission], as there's emerging evidence in all other fields regarding the COVID-19 virus and pandemic. And therefore, we believe we have to be open to this evidence and understand its implication,” professor Benedetta Allegranzi, WHO's technical lead for infection prevention and control, said at the global health body's press briefing on Wednesday.
The WHO's response comes after a group of 239 scientists wrote to the global body, saying that more attention needs to be paid to the airborne route of transmission of COVID-19, and change guidelines accordingly. Dr Maria Van Kerkhove, WHO technical lead for COVID-19, added that they were engaging with many among this group of scientists, including engineers, since they wrote to global health body on April 1 on the subject. “We are working on a scientific brief on the subject that will be out in the coming days,” said Van Kerkhove. “Our focus is on the use of masks for health care workers, and we are looking at the role of airborne transmission in other settings, where you have poor ventilation,” she said.
In their letter, the scientists had pointed out that most public health organisations, including the World Health Organization (WHO), did not recognise airborne transmission except for aerosol-generating procedures performed in healthcare settings. “Handwashing and social distancing are appropriate, but in our view, insufficient to provide protection from virus-carrying respiratory microdroplets released into the air by infected people. This problem is especially acute in indoor or enclosed environments, particularly those that are crowded and have inadequate ventilation relative to the number of occupants and extended exposure periods,” they stated.
Airborne transmission, they say, appears to be the only plausible explanation for several super-spreading events investigated, which occurred under such conditions and others where recommended precautions related to direct droplet transmissions were followed. The evidence for microdroplet transmission, which forms the basis for airborne transmission—viruses released during exhalation, talking and coughing in microdroplets small enough to remain aloft in air and pose a risk of exposure at distances beyond 1-2m from an infected individual—is incomplete, they concede, but that's the case with large droplet and fomite (objects or materials that are likely to carry infection) modes of transmission, too, that are recognised by the WHO. The airborne transmission mechanism operates in parallel with the large droplet and fomite routes that are now the basis of guidance, they state.
So what can one do to counter this mode of transmission? Experts suggest three ways to deal with it—sufficient and effective ventilation (supply clean outdoor air, minimise re-circulating air) particularly in public buildings, workplace environments, schools, hospitals and aged care homes. They suggest that supplementing general ventilation with airborne infection controls such as local exhaust, high-efficiency air filtration, and germicidal ultraviolet lights could also prove to be greatly effective. Besides, avoid overcrowding, particularly in public transport and public buildings. Such measures are practical, easily implemented and many are inexpensive, too.
“For example, simple steps such as opening both doors and windows can dramatically increase air flow rates in many buildings. For mechanical systems, organisations such as ASHRAE (the American Society of Heating, Ventilating, and Air-Conditioning Engineers) and REHVA (the Federation of European Heating, Ventilation and Air Conditioning Associations) have already provided guidelines based on the existing evidence of airborne transmission,” the letter reads.
