THE WORLD HEALTH Organization (WHO) has acknowledged that further study of potential airborne transmission of the coronavirus is needed.
The organisation has published an updated scientific brief on transmission of Sars-Cov-2, the virus that causes Covid-19.
This new report follows the publication of a letter on Monday signed by 239 international scientists who said exhaled droplets under five micrometres in size that contain the virus can become suspended in the air for several hours and travel up to tens of metres.
The comment piece, published in the Oxford Academic journal Clinical Infectious Diseases criticised the WHO for its reluctance to update its advice. The researchers recommended new measures including increasing indoor ventilation, installing high-grade air filters and UV lamps, and preventing overcrowding in buildings and transport.
“There is significant potential for inhalation exposure to viruses in microscopic respiratory droplets (microdroplets) at short to medium distances (up to several metres, or room scale),” wrote the authors, led by Lidia Morawska of the Queensland University of Technology.
“Hand washing 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.”
When an infected person breathes, speaks, coughs or sneezes, they expel droplets of various sizes. Those above five to ten micrometres – which is less than the width of a typical human head hair – fall to the ground in seconds and within a metre or two.
Droplets under this size can become suspended in the air in what is called an “aerosol,” remaining aloft for several hours and travelling up to tens of metres. There has been a debate in the scientific community about how infectious these microdroplets are in the context of Covid-19.
Some studies of particular spreading events have suggested that aerosolisation and microdroplet transmission can happen in a variety of settings.
The air flow from an air conditioning unit appeared to contribute to transmission to customers at several tables in a Chinese restaurant in January, according to a study that appeared in Emerging Infectious Diseases.
Another study that appeared in a report by the Centers for Disease Control and Prevention indicated that the virus was spread by microdroplets from people singing during a choir practice in Washington state in March.
Fifty-three people fell ill at that event and two died.
Busy bars have also emerged as hotspots of contagion, with droplets of all sizes believed to contribute to the spread in these situations.
Cath Noakes, a professor of environmental engineering for buildings at the University of Leeds, who contributed to the paper signed by 329 scientists, said Covid-19 does not spread in the air as easily as measles or tuberculosis, but is a threat nonetheless.
“Covid-19 is more likely to be ‘opportunistically’ airborne and therefore poses a risk to people who are in the same room for long periods of time,” she said.
The authors recognised that the evidence for microdroplet transmission was “admittedly incomplete,” but argued that the evidence for large droplets and surface transmission was also incomplete yet still formed the basis for health guidelines.
“Following the precautionary principle, we must address every potentially important pathway to slow the spread of Covid-19,” they wrote.
Before yesterday, the WHO had advised that the potential for infection from an aerosol occurs “in specific circumstances” mainly in hospitals, for example when a tube is placed down a patient’s airway.
However on Tuesday, following the publication of the letter signed by 239 scientists, Benedetta Allegranzi, the WHO’s technical lead on infection control, told a virtual press conference: “We acknowledge that there is emerging evidence in this field.
We believe that 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.
The updated WHO brief published yesterday evening states that more studies “are urgently needed” to investigate the significance of airborne transmission.
The report still stresses that “to the best of our understanding”, the virus is primarily spread through contact and respiratory droplets.
However it notes: “Under some circumstances airborne transmission may occur (such as when aerosol generating procedures are conducted in health care settings or potentially, in indoor crowded poorly ventilated settings elsewhere).”
The WHO has acknowledged reports related to indoor crowded spaces, including the example of the restaurant in China and the choir practice in Washington, as well as a study of 112 people who attended fitness classes at 12 sports facilities and who were infected.
“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 WHO has now said.
“However, the detailed investigations of these clusters suggest that droplet and fomite transmission [surface contamination] could also explain human-to-human transmission within these clusters.
Further, the close contact environments of these clusters may have facilitated transmission from a small number of cases to many other people (eg, superspreading event), especially if hand hygiene was not performed and masks were not used when physical distancing was not maintained.
Representatives from the National Public Health Emergency Team (NPHET) were asked about the potential significance of airborne transmission at yesterday’s Department of Health briefing.
Dr Cillian De Gascun, director of the National Virus Reference Laboratory in UCD, said we do not have “great evidence” that smaller particles can cause significant spread of the virus.
He said measures implemented in Ireland would not have been successful if this virus was purely airborne and evidence so far indicates that the majority of infections relate to larger droplet transmission.
We know that, typically, with purely airborne viruses the R0 tends to be an awful lot higher, it tends to be up in the teens, which Sars-Cov-2 never was. Typically, we would expect to see the types of reports where people get infected in an empty room. So they go into a room where somebody who had the disease was there previously, that person has gone, the room was apparently empty and people acquire infection in that setting. That’s what happens with airborne pathogens. We’re not seeing that with Sars-Cov-2, it hasn’t been reported.
“Similarly, we hear about stories about ventilation shafts, so you’ve got somebody in a room that’s down the corridor from somebody else, they’ve had no contact, they’ve had no other exposures and the virus has apparently travelled great distances through a ventilation shaft or something like that. And again, that hasn’t been reported,” De Gascun said.
It’s not something that’s that’s a new consideration for us. Droplets and aerosols, the smaller particles, it’s all part of a continuum. At this point, what we know is that the majority of infections do seem to be related to the larger droplet transmission, which is why the physical distancing is so important.
However he said the possibility that airborne transmission contributes to spread in some way cannot be ruled out.
He acknowledged that the smaller, lighter particles take longer to fall to the floor and can travel with air currents. But he added that over the course of time as the particles are suspended in the air, they can evaporate, the virus can become non-viable or they can fall to the floor.
“It just depends on the size and kinds of environmental condition, but in essence from a transmissibility perspective, we don’t have any great evidence that those smaller particles can cause a significant amount of infections over longer distances purely because it hasn’t really been demonstrated. Is it theoretically possible? It is.”
He said the letter published by researcher this week makes references to other viruses, pre-prints and publications that have not been peer reviewed.
“The evidence that we have today’s suggests that it is plausible and theoretically possible. And that’s true – it is – but how much of a driver it is in the context of the global pandemic, we don’t see the evidence yet that it is a significant driver.”
- With reporting from AFP.
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