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WHO changes tack on airborne spread of COVID-19

The World Health Organisation (WHO) has acknowledged "evidence emerging" of the airborne spread of the novel coronavirus, after a group of 239 scientists from 32 countries urged the global body to update its guidance on how the respiratory disease passes between people.

"We have been talking about the possibility of airborne transmission and aerosol transmission as one of the modes of transmission of COVID-19," Maria Van Kerkhove, technical lead on the COVID-19 pandemic at the WHO, is quoted on Reuters as saying.

It was earlier reported that the potential for COVID-19 to spread through airborne transmission by lingering in the air is being underplayed by the WHO and that 239 scientists from 32 countries, in an open letter, were calling for greater acknowledgement of the role of airborne spread of COVID-19 and the need for governments to implement control measures.

The Guardian reports that WHO guidance states that the virus is transmitted primarily between people through respiratory droplets and contact. Aerosol transmission involves much smaller particles that can remain in the air for long periods of time and can be transmitted to others over distances greater than one metre.

Members of the WHO’s infection prevention committee have said that while aerosol transmission may play some role, there is overwhelming evidence that the primary routes of transmission are through direct contact and respiratory droplets expelled during coughing, sneezing or speech. They said introducing new measures to guard against aerosol transmission was unfeasible and unlikely to make much difference to the spread of infection.

The report says the letter is authored by Lidia Morawska, of the Queensland University of Technology in Brisbane, and Donald Milton, of the University of Maryland, and has been endorsed by more than 200 scientists, including some who have been involved in drawing up the WHO’s advice.

They say emerging evidence, including from settings such as meat processing plants where there have been outbreaks, suggests that airborne transmission could be more important than the WHO has acknowledged.

“We wanted them to acknowledge the evidence,” said Jose Jimenez, a chemist at the University of Colorado who signed the paper.

“This is definitely not an attack on the WHO. It’s a scientific debate, but we felt we needed to go public because they were refusing to hear the evidence after many conversations with them,” he said in a telephone interview.

Jimenez said historically, there has been a fierce opposition in the medical profession to the notion of aerosol transmission, and the bar for proof has been set very high. A key concern has been a fear of panic.

“If people hear airborne, healthcare workers will refuse to go to the hospital,” he said. Or people will buy up all the highly protective N95 respirator masks, “and there will be none left for developing countries.”

Jimenez said the WHO panel assessing the evidence on airborne transmission was not scientifically diverse, and lacked representation from experts in aerosol transmission.

Any change in the WHO’s assessment of risk of transmission could affect its current advice on keeping 1-metre (3.3 feet) of physical distancing. Governments, which rely on the agency for guidance policy, may also have to adjust public health measures aimed at curbing the spread of the virus.


The signatories of the letter have recommended three key ways to mitigate the risk of airborne transmission of COVID-19.
1.Ventilation (maximise clean outdoor air, minimise recirculating air) particularly in public buildings, workplaces, schools, hospitals, and aged care homes
2.Airborne infection controls such as local exhaust, high efficiency air filtration, and germicidal ultraviolet lights
3.Avoid overcrowding, particularly in public transport and public buildings
"These are practical and can be easily implemented and many are not costly," Morawska says in an ABC News report.

The scientists say that airborne transmission appears to be the only "plausible explanation" for several superspreading events. Of particular importance was a study (yet to be peer reviewed) of a restaurant in Guangzhou, China, where 10 people from three different families became infected but none of the waiters or 68 other customers did.

Morawska, who was not involved in the study, says the authors observed no evidence of direct or indirect contact between people, but showed how the transmission occurred through the air in a crowded and poorly ventilated space. In that restaurant, there was no outdoor air supply apart from the brief and infrequent opening of a fire door. The researchers found a "re-circulation envelope" formed over the three families' tables, which were in the direct line of one air conditioning unit. All other tables were being serviced by four other air conditioning units.

The study concluded that aerosol transmission had some role in this outbreak, as some customers who became infected were sitting as far as 4.6 metres apart and we know that even a cough only pushes large droplets around 2 metres away. Many studies conducted on the spread of MERS and influenza have also proved the risk of aerosol transmission, Morawska said.

For example, 72% of the 54 passengers on a plane in Alaska which had no ventilation for 4.5 hours contracted influenza in 1977. "There is every reason to expect that SARS-CoV-2 behaves similarly, and that transmission via airborne microdroplets is an important pathway," the open letter says.

According to the report, the WHO said in its latest interim guidance, current evidence suggests COVID-19 is primarily spread through large droplet transmission when people are within 1 metre of each other. It also warns that transmission may occur through fomites (inanimate surfaces or objects) however experts believe that risk is quite low.

The scientists acknowledge there are still parts of the puzzle of evidence missing when it comes to airborne transmission but say we still have much to learn about large droplet transmission too.

They say until we have a vaccine, all routes of transmission must be interrupted and if the importance of ventilation isn't recognised, there will be "significant consequences". "People may think that they are fully protected by adhering to the current recommendations, but in fact, additional airborne interventions are needed for further reduction of infection risk," the scientists say.

They also believe the matter is of "heightened significance now" as countries push ahead with re-opening measures.

Background: The role of aerosols in the transmission of SARS-CoV-2 remains debated. We analysed an outbreak involving three non-associated families in Restaurant X in Guangzhou, China, and assessed the possibility of aerosol transmission of SARS-CoV-2 and characterize the associated environmental conditions. Methods: We collected epidemiological data, obtained a video record and a patron seating-arrangement from the restaurant, and measured the dispersion of a warm tracer gas as a surrogate for exhaled droplets from the suspected index patient. Computer simulations were performed to simulate the spread of fine exhaled droplets. We compared the in-room location of subsequently infected cases and spread of the simulated virus-laden aerosol tracer. The ventilation rate was measured using the tracer decay method. Results: Three families (A, B, C), 10 members of which were subsequently found to have been infected with SARS-CoV-2 at this time, or previously, ate lunch at Restaurant X on Chinese New Year's Eve (January 24, 2020) at three neighboring tables. Subsequently, three members of family B and two members of family C became infected with SARS-CoV-2, whereas none of the waiters or 68 patrons at the remaining 15 tables became infected. During this occasion, the ventilation rate was 0.75-1.04 L/s per person. No close contact or fomite contact was observed, aside from back-to-back sitting by some patrons. Our results show that the infection distribution is consistent with a spread pattern representative of exhaled virus-laden aerosols. Conclusions: Aerosol transmission of SARS-CoV-2 due to poor ventilation may explain the community spread of COVID-19.


[link url=""]Full Reuters report[/link]


[link url=""]Full report in The Guardian[/link]


[link url=""]Clinical Infectious Diseases commentary letter[/link]


[link url=""]ABC News report[/link]


[link url=""]WHO Interim guidance[/link]


[link url=""]MedRxiv[/link]

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