The World Health Organisation (WHO) has acknowledged there is “emerging evidence” that COVID-19 could be spread through particles in the air.
Professor Benedetta Allegranzi, technical lead for infection prevention and control, said the possibility of airborne transmission in “crowded, closed, poorly ventilated” settings could not be ruled out.
It comes after more than 200 scientists wrote an open letter to the agency urging officials to recognise the possibility of airborne transmission of the virus.
Current guidelines from the WHO focus on the virus being spread primarily through droplets from the nose or mouth, which are expelled when an infected person coughs, sneezes or speaks.
The Geneva-based agency advises precautions to avoid these droplets, such as social distancing and regular hand-washing.
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Published in the Clinical Infectious Diseases journal, the letter’s signatories call on health officials to update guidance to include measures which would mitigate the risk of airborne transmission.
The group of 239 experts outline studies which they say suggests coronavirus can spread through smaller particles that linger in the air.
While hand-washing and social distancing are appropriate, the signatories say these measures are “insufficient” in providing protection from “virus-carrying respiratory microdroplets” released into the air by those with COVID-19.
According to the group, measures that should be taken include providing sufficient ventilation, to minimise recirculating air, and avoiding overcrowding, particularly on public transport.
Dr Julian Tang, a consultant virologist and one of the authors of the letter, said that acknowledging that the virus is airborne would drive interventions, "including enhancing our ventilation systems and keeping windows open, being more vigilant with our PPE, and to allow the wearing of N95 masks."
“We understand that all of this cannot and will not happen overnight," comments Tang, "except perhaps the wearing of N95 masks in COVID-19 patient areas and the opening of windows.
“But having the airborne transmission risk acknowledged in the WHO guidelines would motivate and fund hospital teams, governments, NGOs, etc. to work towards these goals, which can also be applied to some forms of public transport – buses, coaches, trains, planes, cruise ships."
When asked about the letter during a press briefing on Tuesday, Prof Allegranzi said officials had been in discussions with the letter’s signatories.
“We acknowledge that there is emerging evidence in this field, as in all other fields, regarding the COVID-19 virus and pandemic,” she told reporters.
“And therefore, 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.”
She added that while airborne transmission could not be ruled out in certain conditions, the evidence was not “definitive” and needed to continue to be gathered.
Speaking at the same press briefing the WHO technical lead on COVID-19, Dr Maria Van Kerkhove, said that many of the signatories of the letter were engineers, which "adds to growing knowledge about the importance of ventilation.
"We have been talking about the possibility of airborne transmission and aerosol transmission as one of the modes of transmission, as well as fomites [objects or materials carrying the infection], fecal-oral, mother-to-child, and animal-to-human of course."
Dr David Nabarro, a special envoy to the WHO on COVID-19, said that while airborne transmission was “certainly part of the puzzle”, it is not the main route of transmission.
“The major way in which that this virus is transmitted is through droplets which come out when you cough, sneeze or even shout,” he told the BBC Radio 4 Today programme. "But there does seem to be the possibility that very small particles containing virus can move further.
“It is certainly part of the puzzle. The key question is just how important is it.
“Right now we think it is not the major means of transmission. The major means of transmission is droplets, which is why we talk about one to two metres distance as being the key thing to avoid spread.”
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Responding to the letter, consultant in infectious diseases at Cambridge University Hospitals Professor Babak Javid said that the suggestions made are "not completely straightforward".
"In many countries where air pollution is a major health hazard, even opening windows does have some consequences, and whilst it’s generally easy to ventilate indoor spaces in clement weather, it’s less practical (or costly both economically and environmentally) in extremes of heat and cold.
"Does that mean children should not attend schools in winter, since no amount of physical distancing is safe if windows have to be closed?"
Prof Javid said he agrees with the letter's signatories that it's possible and "even likely" that airborne transmission will sometimes occur. "It’s not at all clear how common this is, and I agree this is an important and urgent matter to better understand.
"Before invoking the precautionary principle one should acknowledge that no measures come without potential downsides. Zero risk approaches to COVID-19 have major implications for other health and societally important factors (e.g. education).”
Do viruses die?
Strictly speaking, viruses can’t die, for the simple reason that they aren’t alive in the first place. Although they contain genetic instructions in the form of DNA (or the related molecule, RNA), viruses can’t thrive independently. Instead, they must invade a host organism and hijack its genetic instructions.
That said, it makes sense to talk of how long viruses can remain viable and capable of infection. Some – including the influenza virus, and HIV, the virus responsible for AIDS – can’t survive for more than a few hours outside a host organism unless kept under carefully controlled conditions.
But others, notably the deadly smallpox virus, can easily remain infectious for years. Historians now believe that dormant smallpox viruses brought to Australia by British doctors in 1787 could have caused a mass outbreak of smallpox among Aborigines two years later.
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