With the publication of a letter from 239 scientists petitioning the WHO to revise its recommendations to recognize the airborne spread of SARS-CoV-2, the simmering question of SARS-CoV-2 transmission came to a boil again.
At issue is the constantly shifting interpretation of droplet size with reference to SARS-CoV-2.
Traditionally, droplets are defined as large (>5 microns) aqueous bodies. However, airborne (or aerosolized) transmission of the virus has been proposed as a source of infection almost since the inception of the COVID pandemic.
By comparison to droplets, aerosolized particles are infinitesimal. Size alone is not the only important distinction: Droplets fall to earth quickly, but aerosols can travel on air currents potentially for hours. Thus aerosolized viruses are likely to be much more infectious than viruses bound to respiratory droplets, and much more difficult to avoid.
Shortly after publication of the letter, the WHO reiterated its position that SARS-CoV-2 is spread from person to person by droplet-bound virions that fall to earth within a short distance of their source.
Who Is Driving the Droplet vs. Aerosol Transmission Debate?
The coronavirus airborne vs. droplet controversy appears, at this time, to involve scientists with very different perspectives on viral transmission. Engineering professionals involved in the study of airflow in contained environments, and those who research viral infection and spread in populations — such as virologists and infectious disease specialists — are the primary players.
The breach in these communities is evident among the 35 scientists who wrote the commentary for the WHO letter. Among this group, the preponderance were engineering professionals (e.g., civil and construction, mechanical, electrical, environmental). The medical field was represented only by two microbiologists, a virologist, and a pair of pulmonologists.
It is perhaps not surprising that of the three recommendations from the authors, two involve greater investment in ventilation design, infrastructure, and filtration equipment, "particularly in public buildings, workplace environments, schools, hospitals, and aged care homes."
Defining Airborne Transmission
A third, no less vital and no less involved, group of scientists and researchers in the droplet vs. airborne controversy consists of clinicians treating patients with COVID-19.
What the term "airborne" means to these professionals is very different from the interpretation of researchers evaluating the dynamics of airflow, according to Judith O'Donnell, MD, Director of the Department of Infection Prevention and Control, and Chief of Infectious Diseases at Penn Presbyterian Medical Center.
Dr. O'Donnell was the lead author of the Penn Medicine Statement on The Question of Droplet or Airborne Transmission of SARS-CoV-2 issued in late July of this year.
Succinct and straightforward, the statement puts forward the expert opinion of the Penn Healthcare Epidemiologists' Workgroup:
- No study has demonstrated actual clinical evidence of the airborne transmission of SARS-CoV-2;
- The overwhelming majority of transmission of SARS-CoV-2 is via large respiratory droplets as conclusively demonstrated by contact tracing studies, cluster investigations, the lack of infection spread in hospital settings with universal masking protocols and the low estimated R
While acknowledging that poor ventilation indoors can prolong the time that droplets can remain airborne, the statement notes that current ventilation systems in healthcare settings are effective.
"We anticipate more research and guidance from air flow scientists to address whether improving ventilation systems in indoor spaces in the community can make a difference in preventing spread of SARS-CoV-2," Dr. O'Donnell concludes.