10 Reasons Airborne Transmission of SARS-CoV-2 Appears Airtight

Damian McNamara

April 19, 2021

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The scientific evidence for airborne transmission of the SARS-CoV-2 virus from different researchers all point in the same direction — that infectious aerosols are the principal means of person-to-person transmission, according to experts.

Not that it's without controversy.

The science backing aerosol transmission "is clear-cut but it is not accepted in many circles," Trisha Greenhalgh, PhD, told Medscape Medical News.

"In particular, some in the evidence-based medicine movement and some infectious diseases clinicians are remarkably resistant to the evidence," added Greenhalgh, professor of primary care health sciences at the University of Oxford, Oxford, United Kingdom.

"It's very hard to see why, since the evidence all stacks up," Greenhalgh said.

"The scientific evidence on spread from both near-field and far-field aerosols has been clear since early on in the pandemic, but there was resistance to acknowledging this in some circles, including the medical journals," Joseph G. Allen, DSc, MPH, told Medscape Medical News when asked to comment.

"This is the week the dam broke. Three new commentaries came out…in top medical journals — BMJ, The Lancet, JAMA — all making the same point that aerosols are the dominant mode of transmission," added Allen, associate professor of exposure assessment science at the Harvard T.H. Chan School of Public Health in Boston, Massachusetts.

Greenhalgh and colleagues point to an increase in COVID-19 cases in the aftermath of so-called "super-spreader" events, spread of SARS-CoV-2 to people across different hotel rooms, and the relatively lower transmission detected after outdoor events.

Top 10 Reasons

They outlined 10 scientific reasons backing airborne transmission in a commentary published online April 15 in The Lancet:

  1. The dominance of airborne transmission is supported by long-range transmission observed at super-spreader events.

  2. Long-range transmission has been reported among rooms at COVID-19 quarantine hotels, settings where infected people never spent time in the same room.

  3. Asymptomatic individuals account for an estimated 33% to 59% of SARS-CoV-2 transmission, and could be spreading the virus through speaking, which produces thousands of aerosol particles and few large droplets.

  4. Transmission outdoors and in well-ventilated indoor spaces is lower than in enclosed spaces.

  5. Nosocomial infections are reported in healthcare settings where protective measures address large droplets but not aerosols.

  6. Viable SARS-CoV-2 has been detected in the air of hospital rooms and in the car of an infected person.

  7. Investigators found SARS-CoV-2 in hospital air filters and building ducts.

  8. It's not just humans — infected animals can infect animals in other cages connected only through an air duct.

  9. No strong evidence refutes airborne transmission, and contact tracing supports secondary transmission in crowded, poorly ventilated indoor spaces.

  10.  Only limited evidence supports other means of SARS-CoV-2 transmission, including through fomites or large droplets.

"We thought we'd summarize [the evidence] to clarify the arguments for and against. We looked hard for evidence against but found none," Greenhalgh said.

"Although other routes can contribute, we believe that the airborne route is likely to be dominant," the authors note.

The evidence on airborne transmission was there very early on but the Centers for Disease Control and Prevention, World Health Organization (WHO), and others, repeated the message that the primary concern was droplets and fomites.

Response to a Review

The top 10 list is also part rebuttal of a systematic review funded by the WHO and published last month that points to inconclusive evidence for airborne transmission. The researchers involved with that review state that "the lack of recoverable viral culture samples of SARS-CoV-2 prevents firm conclusions to be drawn about airborne transmission."

However, Greenhalgh and colleagues note that "this conclusion, and the wide circulation of the review's findings, is concerning because of the public health implications."

The current authors also argue that enough evidence already exists on airborne transmission. "Policy should change. We don't need more research on this topic; we need different policy," Greenhalgh said. "We need ventilation front and center, air filtration when necessary, and better-fitting masks worn whenever indoors."

Allen agreed that guidance hasn't always kept pace with the science. "With all of the new evidence accumulated on airborne transmission since last winter, there is still widespread confusion in the public about modes of transmission," he said. Allen also serves as commissioner of The Lancet COVID-19 Commission and is chair of the commission's Task Force on Safe Work, Safe Schools, and Safe Travel.

"It was only just last week that CDC pulled back on guidance on 'deep cleaning' and in its place correctly said that the risk from touching surfaces is low," he added. "The science has been clear on this for over a year, but official guidance was only recently updated."

As a result, many companies and organizations continued to focus on "hygiene theatre," Allen said, "wasting resources on overcleaning surfaces. Unbelievably, many schools still close for an entire day each week for deep cleaning and some still quarantine library books. The message that shared air is the problem, not shared surfaces, is a message that still needs to be reinforced."

The National Institute for Health Research, Economic and Social Research Council and Wellcome support Greenhalgh's research. Greenhalgh and Allen had no relevant financial relationships to disclose.

Damian McNamara is a staff journalist based in Miami. He covers a wide range of medical specialties, including infectious diseases, gastroenterology and critical care. Follow Damian on Twitter:  @MedReporter.

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