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Airborne Transmission of COVID-19

Recently,  a team of 239 clinicians, infectious-disease physicians, epidemiologists, engineers and aerosol scientists from 32 countries urged the World Health Organization to acknowledge that COVID-19 can be transmitted through the air, an action  the WHO has vehemently eluded. COVID-19 transmission is the source of a heated, global debate. If indeed airborne transmission is a possible route for the spread of the virus, serious changes in health advice must be made.

How is COVID-19 spread?

Currently, the general consensus is that COVID-19 is transmitted through droplet contact, defined as respiratory droplets carrying infectious pathogens that transmit infection when they travel directly from the respiratory tract of the infectious individual to susceptible mucosal surfaces of the recipient, necessitating facial protection. Influenza and SARS are two examples of diseases where visible droplets can directly transmit from person to person. These droplets typically spread over 1 or 2 meters then quickly fall to the ground or other surfaces that others might touch. Therefore, current hand-washing and physical distancing guidelines aim for people to avoid close contact as much as possible.

What is Airborne Transmission?

Airborne transmission happens through aerosols, droplets that typically measure less than 5 microns in diameter, that people can disseminate by coughing or sneezing, but, even simply through talking, laughing, and singing. These droplets carrying viral particles can become suspended in the air and linger for minutes, or even longer. One study found that aerosols can still maintain infectivity for up to 16 hours. Scientists know that COVID-19 can be spread by asymptomatic people - without coughing or sneezing - and aerosols may explain this phenomenon.

Visual demonstrating airborne transmission (The National Academy of Sciences of the US).

The Evidence

The open letter written by the 239 scientists says multiple studies “have demonstrated beyond any reasonable doubt that viruses are released during exhalation, talking, and coughing in micro-droplets small enough to remain aloft in air.” Researchers have cited several specific events where airborne transmission played a role. This includes an event early in the outbreak when 10 members from 3 families caught COVID-19 while sitting in the same restaurant in Guangzhou, China. None of the people had close contact with each other, but were all sitting in the airflow created by the restaurant’s air conditioning system leading the researchers to conclude that airborne transmission may have been responsible.

Image courtesy of MIT Tech.

Several sensitive laboratory studies paint the complex picture that points to the significance of aerosols as a route of transmission. A study published in May used laser-light scattering to detect droplets produced by healthy volunteers. The authors calculated that one minute of loud speaking generates upwards of 1,000 small, virus-laden aerosols 4 micrometres in diameter that remain airborne for at least 8 minutes. The researchers conclude that “there is a substantial probability that normal speaking causes airborne virus transmission in confined environments”.

Another study published, as a preprint, by Lidia Morawska, aerosol scientist, at the Queensland University of Technology and her colleagues, found that people infected with SARS-CoV-2 exhaled 1,000–100,000 copies per minute of viral RNA, a marker of the pathogen’s presence. Since the volunteers simply breathed out, the viral RNA was much more likely to be present in aerosols rather than large droplets produced by coughing or sneezing. Check out this quick video of Lidia Morawska explaining this.

Outside the lab, it is much more challenging to detect aerosols and prove that they are carriers of a virus. This is partly due to the method of collection of samples. Most devices that suck in air samples delicate the virus’ delicate lipid envelope, making it harder to study virus viability. Julian Tang, a virologist at the University of Leicester, UK who contributed to the commentary, says,  “The lipid envelope will shear, and then we try and culture those viruses and get very, very low recovery.”

However, a few studies have been successful in measuring the viability of the virus in various other settings. Researchers in Wuhan, China found the SARS-CoV-2 virus in aerosol samples collected in a hospital.  A team at the US Department of Homeland Security Science & Technology Directorate in Washington DC found that SARS-CoV-2 in mock saliva aerosols lost 90% of its viability in 6 minutes in summer sunlight, and in 125 minutes in darkness. This research suggests that indoor environments can be especially dangerous since they lack ultraviolet light and the virus can become more concentrated inside.

The WHO has again stopped short of confirming that SARS-CoV-2 spreads through the air but did acknowledge that more research is “urgently needed to investigate such instances and assess their significance for transmission of COVID-19.” Dr. Benedetta Allegranzi, the W.H.O.’s technical lead on infection control remarked, “Especially in the last couple of months, we have been stating several times that we consider airborne transmission as possible but certainly not supported by solid or even clear evidence.”

Julian Tang says the bar of proof is too high regarding airborne transmission. “[The WHO] ask for proof to show it’s airborne, knowing that it’s very hard to get proof that it’s airborne … In fact, the airborne-transmission evidence is so good now, it’s much better than contact or droplet evidence for which they’re saying wash [your] hands to everybody.” Other professionals also agree. Lisa Brosseau, a retired public health professor believes that evidence is trending towards the idea that airborne transmission is “the primary and possibly most important mode of transmission for SARS-CoV-2.”

From the Top

According to Lydia Morawska, strong action from the top is crucial, “Once the WHO says it’s airborne, then all the national bodies will follow.”

Several countries have already recognized that COVID-19 can be spread through aerosols. Japan has long accepted COVID-19’s airborne spread. "If the WHO recognizes what we did in Japan, then maybe in other parts of the world, they will change (their antiviral procedures)," said Shin-Ichi Tanabe, a professor in the architecture department of Japan's prestigious Waseda University.

A graphic designed by the Japanese government in February asking citizens to avoid “the three C’s” - cramped spaces, crowded areas, and close conversation, to minimize the spread of COVID-19 (CBC).

In May, the advice from the German department of health was changed to state that “Studies indicate that the novel coronavirus can also be transmitted through aerosols … These droplet nuclei can remain suspended in the air over longer periods of time and may potentially transmit viruses. Rooms containing several people should therefore be ventilated regularly.” 

This isn’t the first time that the WHO has lagged behind other bodies in updating guidelines. Many clinicians and researchers called on the organization to acknowledge that face masks can help protect the public. The WHO finally made a statement on June 5 and recommended the wearing of cloth masks when social distancing isn’t possible. In contrast, the Centers for Disease Control in the U.S. issued a similar statement on April 3.

What does this mean for public health advice?

If airborne transmission is indeed a possible route for COVID-19 spread, there will be significant changes in precautionary measures. Ventilation will be one of the highest priorities. Researchers would like to see recommendations against the recirculation of air indoors and against the presence of stagnant, dry, stale air. Businesses and institutions are encouraged to invest in better ventilation systems and even sanitation using UV light, since, as mentioned earlier, the virus seems to disappear quickly when exposed to it.

To minimize the risk, please continue to wear masks, wash your hands frequently, and physically distance. In addition, try to spend as much time as possible outdoors and when indoors, aim to keep windows and doors open whenever possible. 

There is a lot more to learn about the SARS-CoV-2 virus and new information is emerging everyday. It is still a complex puzzle. Microbiologist Chad Roy of Tulane University in the US says that airborne transmission can help us solve this mystery because “it may be one reason this is a pandemic, and not simply a small outbreak like any other coronavirus.”

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Featured image courtesy of Getty Images

Article Author: Sara Gehlaut

Article Editor: Olivia Ye