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  • Writer's pictureP.K. Peterson

COVID-19 Airborne Transmission: Hold Your Breath!

“The smaller the creature, the bolder its spirit.” - Suzy Kassem, American poet, writer, poet, and philosopher

“It’s funny, but you never really think much about breathing. Until it’s all you ever think about!”

- Tim Winton, Australian author

The COVID-19 virus has now swept through 213 countries causing almost 13 million cases and leaving over 569,000 dead in its wake. At the heart of this disaster is the mode of transmission of this virus. Can it be transmitted not only by respiratory droplets but also by aerosols (airborne transmission)? The answer is up in the air (pun intended).

Respiratory droplets and protections. Respiratory Droplets are produced during a sneeze or cough, when a cloud of infectious particles >5 microns in size are expelled, resulting in the potential exposure of susceptible persons (those people breathing in the droplets). The force of gravity causes these droplets to rapidly drop to the ground typically within 3 feet of the source person. Examples of pathogens that spread in this manner are rhinoviruses (a major cause of the common cold) and respiratory syncytial virus.

Fairly early on, the World Health Organization (WHO) and the United States Center for Disease Control and Prevention (CDC) recognized that the SARS-CoV-2 virus was spread by respiratory droplets as well as by contact with contaminated surfaces. With knowledge of these two modes of transmission, the WHO and CDC initially recommended frequent handwashing, keeping your hands away from your face, covering your coughs and sneezes, and social distancing (staying at least 6 ft apart). Wearing a surgical or cloth mask was also recommended primarily for symptomatic people to protect others from their respiratory droplets.

Airborne (Aerosol) transmission: size matters. Airborne transmission is the transmission of a virus through aerosols. Aerosols are smaller than respiratory droplets and the term refers to the presence of microbes within particles of <5 microns in diameter. (As a point of reference, about 400 microns would fit in the period at the end of this sentence. SARS-CoV-2 has a diameter of about 0.125 microns.) Aerosols can remain suspended in the air for long periods of time (similar to pollen), can be transmitted to others over distances greater than 1 m (3.2 ft.) and can easily find their way into the lungs.

On July 6 the journal Clinical Infectious Diseases published an electrifying article entitled: “It Is Time to Address Airborne Transmission of COVID-19.” In this article, over 200 scientists from around the world with expertise in airborne transmission of microbes implored the medical community and policymakers to acknowledge that SARS-CoV-2 can spread through the inhalation of aerosols.

This Clinical Infections Diseases article stirred up a fuss at the WHO because it suggests that handwashing and physical distancing aren’t enough to control SARS-CoV-2. While endorsing these measures, the authors of the article recommended providing effective ventilation of indoor areas, including supplying clean outdoor air, minimizing air recirculation, and using high efficiency air filtration as well as germ-killing ultraviolet lights, especially in public buildings, workplaces, schools, hospitals, and nursing homes. They also advise avoiding crowds, especially on public transportation and in buildings.

In response to this publication, on July 9 the WHO issued a new scientific brief summarizing what is known about the different ways SARS-CoV-2 can be transmitted. The WHO acknowledged that there is evidence of airborne (aerosol) transmission but called for more research on the subject.

It is understandable that the WHO is being cautious in recognizing the airborne (aerosol) transmission of SARS-CoV-2 as it has huge implications. The masks we are now wearing would be inadequate because aerosols can penetrate medical masks. Face shields would offer only partial protection due to the gaps between the shield and face. And because aerosols remain suspended in the air for long periods of time and are carried by currents, social distancing of 6 ft. would offer little protection. More important, plans for how to properly open up businesses, restaurants, and schools, as well for how to provide adequate protection in hospitals and long-term care facilities would have to be revisited. Until more research is done, however, we will not know how best to address these issues let alone whether we even need to address them.

The numbers of people infected and killed by SARS-CoV-2 are skyrocketing. On July 10 global daily cases, led by the U.S., reached a new high setting a single-day record for the seventh time in 11 days. In the U.S., active COVID-19 cases have spiked by almost 50% over the past month. This surge of cases prompted National Institute of Allergy and Infectious Diseases director, Anthony Fauci, to recommend that bars, restaurants, and other establishments in states seeing dramatic increases in cases to hit “pause” on their re-openings. He also stated that one major challenge is the nature of the virus itself, which is “spectacularly transmissible.”

Right now, the real Achilles’ heel of controlling COVID-19, are ‘silent spreaders’—asymptomatic patients who transmit the virus through ordinary talking or breathing. They are estimated to transmit 25-50% of all infections.

And then there are the superspreaders—individuals who transmit the virus to far more than the average ‘R-Naught’ (R0) number. (R0 represents the number of new infections from a single case and for COVID-19 is 1.94-5.7.) An example that garnered attention in Minnesota (“Land of 10,000 Lakes and Choirs”) was a choir practice in March in Mount Vernon, Washington where a single infected person passed the virus on to at least 45 other people. (In contrast to people speaking normally, it appears that singers exhale much more air and at higher speeds thereby producing six times the number of aerosol particles.)

To help curtail the transmission of SARS-CoV-2, the CDC now recommends that everyone wear cloth masks when leaving the house for essential trips to public places like grocery stores, work, pharmacies, or mandatory hospital and doctor visits. Some states that are seeing alarming numbers of new COVID-19 cases are requiring that everyone outside wear face coverings, and some businesses are making them mandatory for entry or service. (N95 filtering facepiece respirators, which remove 95% of very small [0.3 micron] particles from the air are prioritized for healthcare workers.)

Right now, taking airborne (aerosol) transmission seriously should be factored into the reopening of businesses, restaurants and, in the near future, schools. Short of all of us holding our collective breath (or the release of an effective vaccine) this could make a big difference in slowing the spread of this spectacularly transmissible enemy.

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Main Page images courtesy of Shuxian Hu, MD. Dr. Hu is a scientist in the Neuroimmunology Research Laboratory at the University of Minnesota.

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