The COVID-19 spread: What do we know?

Taking my dog out for a walk, I’ve noticed a new etiquette. As I cross paths with people, they are not making eye contact. I realise that, like me, they don’t want to be caught in the act of holding their breath. The World Health Organization may not agree, but it seems the public has come to the conclusion that the COVID-19 virus is airborne.

The official WHO line is that COVID-19 is spread through a shower of droplets when an infected person coughs or sneezes. Being heavy they drop to the surface, but like little cannon balls they might trace a trajectory of up to a metre and a half before they do. So if we keep a distance of 1.5 metres and wipe surfaces, we should be OK.

But what if the virus is not just spread by droplets? What if buoyant clouds of air bearing virus particles are released, not just by the coughers and sneezers, but by asymptomatic people who are simply talking? 

While contagion at weddings and parties isn’t surprising, other events are harder to explain – such as last month’s outbreaks in the wake of a biotech conference in Boston, or the rampant contagion in quarantined cruise ships.

Combining these bits of evidence, it becomes very tempting to join the dots: airborne virus could be explaining some of the spread. For the last week, as I researched this story, I hesitated to do so. But as Lidia Morawska, the Director for the International Laboratory for Air Quality and Health, based at QUT, told me on Friday, “You should join the dots. There’s been a huge and growing frustration among scientists working in this field.” 

“We cannot reject the possibility that it is in the air.”

The frustration lies with the fact that WHO is clinging to a 90-year-old medical dogma. The droplet dogma, articulated by William Wells in 1930 for tuberculosis, holds that contagion is largely limited to the distance covered by droplets that are larger than five to 10 microns in size. “That’s why hospital beds are a metre apart,” says Kanta Subbarao, based at Melbourne’s Doherty Institute and the director of the WHO Collaborating Centre for Reference and Research on Influenza.

Several types of studies now challenge that view.

Some come from scientists like Morawska and Lydia Bourouiba at MIT – both physicists who turned to the study of infectious disease after the 2003 SARS epidemic. Their studies have shown that infected people don’t just spray heavy droplets – they also exhale turbulent clouds that carry airborne droplets of various sizes, some smaller than five microns. 

Bourouiba has shown that turbulent clouds could travel up to eight metres. And it doesn’t necessarily take a cough or sneeze. People exhale wafting clouds as they talk; the louder they speak the further the cloud goes. Moreover, according to William Ristenpart and colleagues at the University of California, some people release 10x more particles than others. The fuzzy notion of ‘superspreaders’, people who infect others way beyond the average of two, might instead be ‘speech super-emitters’, they suggest.

Morawska’s studies have shown that the pseudomonas bacteria, coughed up by patients with cystic fibrosis, could spread four metres and survive for 45 minutes. And there is evidence that viruses such as SARS can infect people via an airborne route.

A 2003 paper in The New England Journal of Medicine documented a flight in which one SARS case led to 16 possible infections – eight of whom were sitting within three rows of the symptomatic patient. Another study the same year suggested SARS transmission through an air shaft in a housing complex in Hong Kong.

There’s growing evidence that the current SARS virus – SARS–CoV-2 – the cause of COVID-19, is also airborne. Across the globe, anecdotal reports are emerging. On 30 March, the LA Times reported on a Washington state case: “When choir practice turned fatal”. Of 60 choristers who turned up for practice, 45 were infected despite hand sanitising and foregoing hugs. Three were hospitalised and two died.

There’s also a growing body of research. One widely cited study suggesting that artificially created aerosols are infectious, comes from the US Centers for Disease Control and Prevention (CDC). Researchers generated aerosols of COVID-19 virus by squirting virus-containing fluids into a rotating drum. They found that air sampled from these drums could infect cells in a dish. The virus remained airborne and infectious for three hours. (The same study also found that infectious virus lasted on plastic and stainless steel surfaces for up to three days. On cardboard it was one day.)

Other reports have detected small quantities of airborne virus in hospitals or public places but have not shown whether those shreds of virus are infectious. One study (circulating as a preprint and still subject to peer review) came from Wuhan. Ke Lan, the director of the State Key Laboratory of Virology at Wuhan University, and colleagues tested air samples from an established hospital, Renmin, and from a field hospital.

They barely detected airborne virus at the established hospital, even in the intensive care rooms “suggesting the negatively pressurised isolation and high air exchange rate in these buildings are very effective in containing the airborne transmission of SARS-CoV-2”, said Lan in an email. 

But small quantities were picked up in the field hospital in a medical staff area where contaminated clothing was removed and in a patient toilet; faeces are known to carry the virus and it’s possible flushing helps render it airborne. But two public areas that were prone to overcrowding also returned positive air samples: the entrance to a department store and outside Renmin Hospital. A similar finding of detectable airborne virus in hospital rooms, published as a preprint, came from Joshua Santarpia and colleagues at the University of Nebraska Medical Centre.

WHO was unswayed by both these studies. Their view was that the infectious aerosol tested by the CDC researchers was “artificially generated” and did not reflect what is likely to happen in hospitals or outdoors. As for the airborne virus found in hospitals and public places in Wuhan, there was no evidence it was infectious. Indeed, just a few days ago the WHO reaffirmed its position with an emphatic tweet. “COVID-19 is not airborne.”

For Kowarska, the WHO position is no longer tenable. “We cannot reject the possibility that it is in the air. We have to assume that it is.”

The scientists of the US National Academy of Science, Engineering and Medicine agree. A letter penned on 1 April to White House science adviser Kelvin Droegemeier concluded, “While the current SARS-CoV-2 specific research is limited, the results of available studies are consistent with aerosolisation of virus from normal breathing.”

So does Wuhan University’s Ke Lu. “Based on our observation, patients or infected carriers flushing the toilet without putting the toilet seat down, or sneezing, coughing or talking face-to-face are undoubtedly high-risk behaviours which possibly generate SARS-CoV-2 aerosols.” 

“We don’t want pandemonium in the midst of the pandemic.”

In many parts of the world, and certainly amongst my fellow pedestrians, it seems that we are already operating on the assumption that COVID-19 virus is in the air. We are advised to shelter at home in our cordon sanitaire. Outside we’re also commanded to keep our distance. 

That keeps us pretty safe, believes Kowarska. Even if there are virus particles floating about, they’re highly diluted. Like a cigarette smoker’s puff of smoke, they quickly dissipate. 

What Renaissance-era Swiss physician Paracelsus said about medicines is true for viruses: the dose makes the poison. The minimum infectious dose for the original 2003 SARS virus was estimated by one paper at between 16 to 280 particles.

The highest estimate of airborne virus from the Wuhan study was 16-42 copies per square metre in the non-ventilated changing room of the field hospital. “We think, if these are infectious viruses, 16 to 42 copies are possible to cause infection,” Lan emailed on Sunday.  And what of the 11 particles per cubic metre detected outside the department store? Lan cautioned. “In the real world we know its viability is apparently impacted by many factors such as temperature, humidity, the particle size, the velocity etc.”

Other evidence suggests spread is not due to open air transmission. For instance, tracking studies in Guangdong province about 1000 kilometres south of Wuhan, showed that most people acquired their infection from close contact either within families, in jail or as healthcare workers. Bruce Aylward, the WHO assistant Director General, points out that of 320,000 tests carried out at the peak of the outbreak, only 0.47% were positive. If people were catching it from the air, the epidemic would have looked quite different, suggests Sharon Lewin, Director of Melbourne’s Doherty Institute.

“If aerosol spread was really a big contributor to spreading infection, I would expect a far higher frequency of infections in people who have no known exposures. This of course does happen, but I suspect this happens infrequently.’’

So outside exposure is probably fine. But transmission in a static crowd, such as the one witnessed at Sydney’s Bondi beach in mid-March, could be a different matter. And what happens in enclosed spaces like the supermarket? For Kowarska, the key thing is to heed Paracelusus and minimise the dose. The supermarket should be well ventilated and use fresh not recirculated air. It should restrict the number of people. And you, the shopper, should be in there for no more than 10 minutes, she says. 

The real problem is for those who can’t minimise their exposure. And that brings us to the vexed issue of masks. There are two types. The N-95 mask, which forms a seal against the face and blocks 95% of particles smaller than three microns, is effective against airborne virus. Then there are loose-fitting surgical masks, which are designed to protect the wearer from falling droplets but have less filtering capability. 

A paper from Nancy Leung at the University of Hong Kong and colleagues published on 3 April provides long-awaited evidence that surgical masks could block exhalations of seasonal coronavirus from infected people. Although they did not specifically test this season’s COVID-19 virus, the authors claim: “This has important implications for control of the COVID-19 suggesting that surgical face masks could be used by ill people to reduce onward transmission.”

For Euan Tovey, an expert on airborne allergens and virus at Sydney University, it’s impressive data: “Such experiments are extremely hard to do well.” But what concerns him most is that just by breathing, the people in this study were spreading the virus into the air.   “I think masks have a role in an overall strategy,” he says. “We will not have proof until we do the detailed studies, but that around a third of people who were only breathing produced a small virus aerosol sends a slight shiver down my spine.

Related reading: Is COVID-19 spread through the air?

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