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The scientific evidence for airborne transmission of the SARS-CoV-2 virus from different researchers all points in the same direction – that infectious aerosols are the most important means of person-to-person transmission, according to experts.
Not that it is without controversy.
The science that supports aerosol transfer “is clear, but not accepted in many circles,” said Trisha Greenhalgh, PhD. Medscape Medical News
“Particularly some in the evidence-based medicine movement and some infectious disease clinicians are remarkably resistant to the evidence,” added Greenhalgh, professor of primary health sciences at the University of Oxford, Oxford, UK.
“It’s very difficult to see why, as the evidence is all piling up,” Greenhalgh said.
“The scientific evidence about dispersion of both near-field and far-field aerosols has been clear since the start of the pandemic, but there has been resistance in some quarters to recognize this, 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 comments came out … in top medical journals – BMJ, The Lancet JAMA – all with the same point that aerosols are the dominant mode of transmission, ”added Allen, associate professor in the science of exposure assessment at Harvard TH Chan School of Public Health in Boston, Massachusetts.
Greenhalgh and colleagues point to an increase in COVID-19 cases in the wake 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 supporting air transmission in a commentary published online April 15 The Lancet
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The dominance of airborne transmission is supported by long-range transmission seen in super spreaders.
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Long-distance transmission between rooms has been reported in COVID-19 quarantine hotels, environments where infected people have never spent time in the same room.
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Asymptomatic individuals are responsible for an estimated 33% to 59% of SARS-CoV-2 transmission and could spread the virus through speech, which produces thousands of aerosol particles and some large droplets.
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Transfer outside and in well-ventilated indoor areas is lower than in confined spaces.
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Nosocomial infections are reported in healthcare facilities where protective measures target large droplets but not aerosols.
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Viable SARS-CoV-2 has been detected in the air of hospital rooms and in the car of an infected person.
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Researchers found SARS-CoV-2 in hospital air filters and building ducts.
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It’s not just humans – infected animals can infect animals in other cages that are only connected through an air duct.
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No strong evidence disproves airborne transmission, and contact locating supports secondary transmission in crowded, poorly ventilated indoor areas.
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Only limited evidence supports other means of SARS-CoV-2 transmission, including via fomites or large droplets.
‘We thought we would sum it up [the evidence] to clarify the arguments for and against. We searched hard for evidence against it, but found none, “Greenhalgh said.
“While other routes may contribute, we think the airway is likely to be dominant,” the authors note.
Evidence of airborne transmission was very early, but the Centers for Disease Control and Prevention, the World Health Organization (WHO), and others reiterated the message that the primary concern was droplets and fomites.
Response to a review
The top 10 list is also partly a rebuttal of a systematic review funded by WHO and published last month that points to unclear evidence for air transmission. The researchers involved in that review state that “the lack of recoverable SARS-CoV-2 viral culture samples prevents firm conclusions from being drawn about airborne transmission.”
However, Greenhalgh and colleagues note that “this conclusion, and the widespread dissemination of the review’s findings, is of concern because of its implications for public health.”
The current authors also state that sufficient evidence already exists about air transmission. “Policies should change. We don’t need more research on this topic; we need different policies,” Greenhalgh said. “We need front and center ventilation, air filtration if necessary, and better fitting masks worn indoors.”
Allen agreed that counseling has not always kept pace with science. “With all the new evidence gathered about air transmission since last winter, there is still widespread public confusion about the modes of transmission,” he said. Allen also serves as a Commissioner of The Lancet COVID-19 Committee and Chair of the Committee’s Task Force on Safe Work, Safe Schools, and Safe Travel.
“It was only last week that CDC pulled out of the ‘deep cleaning’ guidelines and instead rightly said that the risk of touching surfaces is low,” he added. “Science has been clear on this for over a year, but official guidelines have only recently been updated.”
As a result, many companies and organizations continued to focus on ‘hygiene theater,’ Allen said, ‘by wasting resources on over-cleaning surfaces. the message that shared air is the problem, not shared surfaces, is one that 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. It covers a wide variety of medical specialties, including infectious diseases, gastroenterology and intensive care. Follow Damian on Twitter: @MedReporter
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