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Explained: 10 key reasons why coronavirus transmission is primarily airborne

A team of experts has looked at available research and published their assessment that there is strong, consistent evidence that the primary transmission route of SARS-CoV-2 is indeed airborne. What are the implications of the assessment?

At Thane Railway Station on Friday. (Express Photo: Deepak Joshi)

Since last year, several studies have found that the coronavirus SARS-CoV-2 spreads mainly through the air. Yet there have also been other studies, including a recent one funded by the World Health Organization, that have found the evidence inconclusive.

Now, a team of experts has looked at available research and published their assessment in The Lancet: that there is strong, consistent evidence that the primary transmission route of SARS-CoV-2 is indeed airborne.

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What are the implications of the assessment?

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If transmission is airborne, public health measures would need to take that into account. Measures that focus solely on large-droplet-borne transmission, but fail to treat the virus as predominantly airborne, would leave people unprotected.

“We need to place less emphasis on deep cleaning and repeated hand-washing (but still follow basic hygiene measures of course),” Dr Trisha Greenhalgh of the University of Oxford, lead author of the paper, told The Indian Express, by email. “We need to put ventilation front and centre (e.g opening windows, carbon dioxide monitors); air filtration when necessary; better-fitting masks worn whenever indoors; and attention to what the Japanese call the 3Cs: avoid close contact, crowded places and closed [poorly-ventilated] spaces,” she said.

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How did the experts reach this conclusion?

Reviewing existing research, the six experts from the UK, US and Canada identified 10 streams of evidence that collectively support the hypothesis that SARS-CoV-2 primarily transmits through the airborne route.

1. Super-spreading events account for substantial SARS-CoV-2 transmission. Indeed, the authors wrote, such events may be the pandemic’s primary drivers. Detailed analyses of human behaviours and other variables in concerts, cruise ships etc have shown patterns “consistent with airborne spread of SARS-CoV-2 that cannot be adequately explained by droplets or fomites”, they wrote.

2. Long-range transmission of SARS-CoV-2 between people in adjacent rooms has been documented in quarantine hotels, but never in each other’s presence.

3. Asymptomatic or pre-symptomatic transmission from people who are not coughing or sneezing is likely to account for at least a third, and perhaps up to 59%, of all transmission globally and is a key way SARS-CoV-2 has spread around the world, indicating a predominantly airborne mode of transmission.

4. 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 authors wrote.

5. New infections have been documented in healthcare organisations where there have been strict contact-and-droplet precautions and use of PPE designed to protect against droplet but not aerosol exposure.

6. Viable SARS-CoV-2 has been detected in the air. In laboratory experiments, SARS-CoV-2 stayed infectious in the air for up to 3 hours. In one study, viable SARS-CoV-2 was identified in air samples from rooms occupied by Covid-19 patients in the absence of aerosol-generating procedures; in another study, it was detected in air samples from an infected person’s car.

7. SARS-CoV-2 has been identified in air filters and building ducts in hospitals with Covid-19 patients; such locations could be reached only by aerosols.

8. Studies involving infected caged animals that were connected to separately caged uninfected animals via an air duct have shown transmission of SARS-CoV-2 that can be adequately explained only by aerosols.1

9. No study “to our knowledge”, the authors wrote, has provided strong or consistent evidence to refute the hypothesis of airborne SARS-CoV-2 transmission. Some people have avoided SARS-CoV-2 infection when they have shared air with infected people, but this situation could be explained by a combination of factors, including variation in the amount of viral shedding between infectious individuals and different environmental conditions.

10. There is limited evidence to support other dominant routes of transmission—ie, respiratory droplet or fomite.

Since all this is from existing research, wasn’t it already known?

Alongside all the research that pointed to airborne transmission as the primary route, other studies have found the evidence inconclusive. In July last year, a team of over 200 scientists had written to the WHO about airborne transmission; the WHO later agreed that airborne transmission “cannot be ruled out” in certain situations, such as choir practice or in restaurants.

The most recent WHO-funded study, currently published on a preprint server, has found that “the lack of recoverable viral culture samples of SARS-CoV-2 prevents firm conclusions to be drawn about airborne transmission”.

The assessment in The Lancet quotes this conclusion and says it is concerning because of the public health implications.

“What we’re saying is that recoverable viral cultures are only one element of the evidence base (and we also did find studies which had shown recoverable viral cultures from the air),” Dr Greenhalgh told The Indian Express. (See argument No. 6 above)

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