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, said in an interview.
“In particular, some in the evidence-based medicine movement and some infectious diseases clinicians are remarkably resistant to the evidence,” added Dr. Greenhalgh, professor of primary care health sciences at the University of Oxford (England).
“It’s very hard to see why, since the evidence all stacks up,” Dr. 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 this news organization 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 Dr. Allen, associate professor of exposure assessment science at the Harvard T.H. Chan School of Public Health in Boston.
Dr. 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:
- The dominance of airborne transmission is supported by long-range transmission observed at super-spreader events.
- Long-range transmission has been reported among rooms at COVID-19 quarantine hotels, settings where infected people never spent time in the same room.
- Asymptomatic individuals account for an estimated 33%-59% of SARS-CoV-2 transmission, and could be spreading the virus through speaking, which produces thousands of aerosol particles and few large droplets.
- Transmission outdoors and in well-ventilated indoor spaces is lower than in enclosed spaces.
- Nosocomial infections are reported in health care settings where protective measures address large droplets but not aerosols.
- Viable SARS-CoV-2 has been detected in the air of hospital rooms and in the car of an infected person.
- Investigators found SARS-CoV-2 in hospital air filters and building ducts.
- It’s not just humans – infected animals can infect animals in other cages connected only through an air duct.
- No strong evidence refutes airborne transmission, and contact tracing supports secondary transmission in crowded, poorly ventilated indoor spaces.
- 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,” Dr. 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, and others repeated the message that the primary concern was droplets and fomites.