The latest coronavirus outbreak from hotel quarantine in Western Australia has focused attention on how the virus is transmitted. Hotel quarantine is supposed to be secure with returned travellers separated from each other. Staff and residents are meant to be regularly tested for Covid-19 and infection control procedures are supposed to be rigorous.
Yet as Burnet Institute epidemiologist Prof Mike O’Toole points out, there have been 14 hotel quarantine leaks in Australia since November. So if infection control procedures are in place and people are kept apart, how is the virus that causes Covid-19 escaping?
A number of experts, including epidemiologists and infectious diseases physicians, say the role of airborne transmission has been underestimated.
It prompted Australia’s chief medical officer, Prof Paul Kelly, to tell a Senate inquiry on Tuesday night: “There is no question, and never has been a question, right throughout this pandemic that aerosols do play a part in the transmission of this virus.”
This was particularly the case indoors when many people were positive with the virus and in places with inadequate ventilation – such as some hotel quarantine facilities. But, Kelly said, it was not the key form of transmission.
“This idea that the commonwealth government … are all denying that aerosols are important is ridiculous and false,” he said.
What is airborne transmission?
According to the World Health Organization, droplets less than five micrometers in diameter are aerosols. By comparison, droplets expelled through the air when people cough and sneeze are referred to as respiratory droplets. They are usually larger in size, do not linger in the air, spread when people are in close contact with each other, and can drop on to and contaminate surfaces, including hands.
The WHO says current evidence suggests the virus is primarily transmitted between people via these larger respiratory droplets and the surfaces they contaminate.
The advice does not state that Covid-19 is never spread by airborne transmission, just that it appears much less common. The WHO has been examining whether airborne spread might be more common in facilities with poor ventilation. There are a number of ways the WHO theorises airborne transmission may occur. This includes respiratory droplets evaporating to generate microscopic aerosols.
“Normal breathing and talking results in exhaled aerosols,” the WHO states. “Thus, a susceptible person could inhale aerosols and could become infected if the aerosols contain the virus in sufficient quantity to cause infection within the recipient. However, the proportion of exhaled droplet nuclei or of respiratory droplets that evaporate to generate aerosols, and the infectious dose required to cause infection in another person, are not known.”
Singing and shouting can also generate aerosols. Certain medical procedures can create aerosols – a danger when performed on Covid-positive patients.
A review of existing studies into airborne Covid-spread published in March and partially funded by the WHO found there was still “no firm conclusions to be drawn about airborne transmission”.
Could airborne transmission be playing a larger role than anticipated?
Australia’s Infection Control Expert Group, an independent group of medical and scientific experts that advise the federal government, issued a statement on 21 January that it still believes the virus “is mainly transmitted by close personal contact (via respiratory particles)” or via contaminated surfaces.
“It is less likely that transmission occurs via small respiratory particles (aerosols) that remain suspended in the air for prolonged periods,” the advice said. “Airborne transmission is believed to mainly occur because of specific procedures or behaviours, in particular in poorly ventilated, crowded indoor settings.”
Iceg member Prof Peter Collignon, an infectious diseases physician, said while he could not speak on behalf of the expert group it reviewed evidence “all the time” and he personally did not believe there was enough compelling evidence to change the advice.
He said while some people had differing definitions of aerosols and what constituted airborne spread, the main point was it meant particles stayed in the air for hours and travelled more than 10 metres.
“Ordinary masks, like cloth masks and surgical masks, don’t work very well against these aerosols,” he said. But, Collignon said, there was strong evidence ordinary masks had significantly reduced spread in communities and high-risk settings such as healthcare and aged care when worn properly suggesting respiratory spread.
He said in some cases in hotel quarantine there had been clear breaches of personal protective equipment procedures including mask-wearing.
“When I look at the hotels where you’ve got people infected leaking infections from there, there are staff or other people not wearing protective equipment with either mask or eye protection missing, or they’ve become infected after staying in a room on the opposite side of a corridor where both doors have been open at the time.
“Corridor spread has got to do with the ventilation of the corridor, and engineers have identified some of these hotels where breaches occur aren’t fit for purpose and don’t have adequate ventilation.”
Collignon said if aerosol spread was a significant issue, people would have to wear special respiratory masks with a very close facial fit known as N95 masks.
“There’s not a lot of people that seem to get infected if they walk into a restaurant or on to a bus two hours after somebody else with an infection has already been there,” Collignon said. “I do not think widespread airborne spread is supported by adequate data, and the implications would be huge because it would mean the standard masks people wear in the community are ineffective.”
What are other experts saying?
A preprint paper published in the April edition of the Medical Journal of Australia states that thinking airborne transmission only happens in certain settings, such as during aerosol-generating medical procedures such as intubation, is outdated.
Led by the epidemiologist and biostatistician with the University of Western Australia, Dr Zoë Hyde, the article states Australia must do more to recognise and prevent aerosol and airborne transmission. There were now enough examples, the authors wrote, including an outbreak in an apartment complex in South Korea. Only residents living in apartments connected by a common ventilation shaft were infected. An investigation found no other possible contact between the cases than the airborne infection through a single air duct in the bathroom.
“A similar outbreak occurred in an apartment building in China, in which the virus appeared to spread from persons on the 15th floor to persons in vertically aligned flats on the 25th and 27th floors … faecal aerosols produced by toilet flushing were thought to be responsible,” the MJA article said.
“There have also been several documented outbreaks in healthcare settings which were not only highly suggestive of airborne transmission but also demonstrated that physical distancing and the use of surgical masks is not always sufficient to prevent infection.”
Meanwhile, a piece in medical journal the Lancet also published in April outlines “10 scientific reasons” in support of airborne transmission. It described the findings from the WHO-funded review into airborne spread as “concerning”.
“It is a scientific error to use lack of direct evidence of Sars-CoV-2 in some air samples to cast doubt on airborne transmission while overlooking the quality and strength of the overall evidence base,” the Lancet article, led by a British professor of primary health care, Trisha Greenhalgh, said.
“Transmission of Sars-CoV-2 is higher indoors than outdoors and is substantially reduced by indoor ventilation. Both observations support a predominantly airborne route of transmission.”
The piece also says spread has occurred in healthcare settings with strict physical distancing measures and precautions in place to prevent droplet spread including use of personal protective equipment.
Where does that leave us?
Expert opinion is divided. There have been calls from the Public Health Association of Australia and others for the Iceg to update its advice on airborne transmission.
The PHAA president, Adjunct Prof Tarun Weeramanthri, a public health doctor who led an independent review of hotel quarantine in WA, wrote in an article for health publication Croakey: “The biggest problem is the failure to accept and simply state what is now obvious from the science and first-hand analysis of the cases – the significance of airborne transmission generally, and in the hotel quarantine environment specifically.”
Meanwhile, the WHO says – as with many aspects of Covid-19 – that “more research is needed” given the possible implications of such a route of transmission, but that “to the best of our understanding, the virus is primarily spread through contact and respiratory droplets”.
The president of the Australian Medical Association, Dr Omar Khorshid, told a Senate inquiry into the government’s handling of the pandemic on Tuesday evening that “it has been extremely frustrating to watch this” transmission debate.
“It’s almost been a pitched battle between people on either side of an argument,” he said.
“Actually, as is often the case, they’re both right. There’s no doubt that if you look at the epidemiology of Covid illness, it is droplet spread in most circumstances. If it was mostly spread by aerosol spread, it would be like measles and everybody who walks into a room would get it and we’d all have it by now.
“However, there’s no black and white here, and there’s no doubt that airborne transmission has been implicated in a whole series of outbreaks all over the world, in breaches of quarantine, and breaches in hospital and health care settings.”
Khorshid said it was critical to ensure good protocols were in place to protect people in the limited circumstances in which aerosol spread could occur. He said the National Covid-19 Clinical Evidence Taskforce was expected to update its guidelines to protect against breaches involving aerosol spread.
“Yes, in certain circumstances, and only limited circumstances, it is airborne spread, and therefore in order to protect people, in order to have a system that works 99.99% of the time, you need to put in appropriate levels of protection,” Khorshid said.