This was published 4 years ago
Scientists divided over key coronavirus question
A world expert in aerosols says the new coronavirus may be spreading more easily through the air than first thought - from breathing as well as coughing or sneezing in some cases.
While scientists agree the virus is not airborne in the same way as other infectious diseases such as measles, they are split on the question of how big a risk it poses in the air.
The World Health Organisation says there’s not enough evidence to say the virus can jump from person to person in small or aerosolised particles. It’s mostly shed by larger water droplets from the nose and mouth, tiny balls of mucus, salt and virus that can shoot out up to about a metre when an infected person coughs or sneezes. Sometimes they land on and contaminate surfaces, but they’re too heavy to survive long in the air.
The question of how easily the new coronavirus can spread in the air is still up for debate.Credit: AP
A growing number of scientists, including infectious disease expert Professor Raina MacIntyre and aerosol scientist and WHO advisor Professor Lidia Morawska, now say the risk from aerosols may have been underestimated. Warning signs are piling up, Professor Morawska says - the virus rips through a cruise ship even after passengers are isolated in their cabins, a choir meets in Washington and 45 out of the 60 singers leave the two hour rehearsal infected even though none have symptoms.
As an Australian Department of Health spokeswoman also noted, viruses do not always fall neatly into either aerosol or droplet. They can leave the body as both.
“It’s old medical dogma from the 1930s that they only travel a metre,” Professor Morawska said. “But we know more now. We've already shown other viruses like the flu [that mostly] shed in droplets can also spread from breathing. [In a] pandemic, we need to assume the worst."
That still doesn't mean you'll catch the virus from passing someone on the street "unless they coughed right in your face", Professor Morawska says. Like cigarette smoke, the virus will disperse in open spaces. And, just like smoke, it can build up in enclosed areas without ventilation.
"We don’t know how much you would have to inhale to get infected, the smaller particles at least carry less virus, but it’s possible," she said.
The Department of Health said COVID-19 was "not as efficient" at spreading in the air as other diseases considered to be airborne - a greater risk was posed by the "underestimated" spread of COVID-19 through touching contaminated surfaces.
Other experts such as infectious disease physician Associate Professor Sanjaya Senanayake say the virus is unlikely to be spreading frequently in the air. If it were, the shape of the pandemic would likely look different- spreading faster and amongst more people without close contact to known cases.
On paper, early studies into the new virus show mixed results - some found it in the air of hospitals treating patients, some didn’t. Even if the virus stays airborne, that doesn’t necessarily mean it stays infectious, Professor Morawska noted, as all viruses start to die once they leave the body.
One laboratory study by scientists from the US Centers for Disease Control and Prevention detected the virus in the air for up to three hours, but it did not reflect real-world conditions. Still, America’s National Academies of Sciences, Engineering and Medicine wrote to the White House earlier this month saying the current research supports the possibility COVID-19 can be spread by aerosols from breathing, as was observed to a degree during the first dangerous coronavirus outbreak, SARS.
But back then in 2003, Associate Professor Senanayake notes the virus mostly aerosolised in hospitals where treatments such as intubation and ventilation increase the risk. The WHO has warned healthcare workers this may happen again and many wards treating COVID-19 already operate as if it's airborne, including in Australia.
"We're all following that really closely and it seems to be holding up well," Associate Professor Senanyake said.
Both the WHO and the Australian government say masks are still only necessary for people with symptoms and those treating them, and must not be wasted by the general public as shortages of the product could put frontline workers at risk.
But in the US, the CDC has reversed its own advice, now urging people to wear cloth masks whenever they are somewhere poorly ventilated or can’t keep their distance from others, such as in a supermarket aisle. Its guidance to healthcare workers also states that, while the extent of aerosol transmission is still unclear, the virus can spread when someone "coughs, sneezes or talks".
Professor Morawska agreed with the masks push, adding there should be more focus on the need for natural ventilation by throwing windows open wherever possible rather than using air circulation systems. She is now working with scientists from around the world to write enhanced guidelines for aerosol transmission.
Associate Professor Senanayake said air conditioning and heating could still be considered safe in most settings as droplets will not travel that far. And the department agreed that, while there was not enough data on the question, the risk from ventilation outside hospitals.
A spokeswoman said Australia's advice on masks had not changed on Thursday but the evidence on transmission was being reviewed all the time by the medical experts advising government.
Professor MacIntyre and Associate Professor Senanayake said wearing masks in public was not so urgent for Australia compared to New York where there was rampant community transmission.
“Perhaps somewhere like Sydney, where they’ve had local outbreaks they could help," Associate Professor Senanayake says. "But everyone just rushing out and wearing a mask won’t."
NSW Health said air conditioners and heating were considered safe to use even when people in the home had COVID-19.
"What is important for suspected and confirmed cases is to try and stay in a room separate to the rest of their family and if possible, use a different bathroom...to avoid the spread of COVID-19 through droplets...and surface contamination," a spokeswoman said. "It is only in the very sickest patients, who are in our specialist ICUs, or patients who require hospital treatments like intubation, where we are likely to see the virus be airborne."