September 28, 2020

The keys to understanding the debate on the role of aerosols in the spread of coronavirus

The COVID-19 pandemic has given rise to intense scientific debates that, far from being relegated to the research field, have crossed borders and laboratories to reach the general population and the media around the world. The appearance of a new virus, such as SARS-CoV-2, has been surrounded by a multitude of unknowns and unknown data that complicated and continue to complicate the taking of epidemiological and sanitary measures. Therefore, scientific activity has been key to advancing knowledge of the coronavirus and making the best decisions. However, this process is far from being error-free and from always being consensual among all scientists. When knowledge about a subject is limited, different positions and scientific discrepancies often arise. In any case, science progresses through questioning the different hypotheses by conducting more studies and experiments.

Airborne transmission of COVID-19: the question is to find out how far to separate

Airborne transmission of COVID-19: the question is to find out how far to separate

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In that sense, the most heated scientific debate in this pandemic was over the utility of masks in the general population. Although there are now multiple tests of their effectiveness in limiting coronavirus infections and health authorities agree to recommend them, just a few months ago institutions such as the World Health Organization (WHO) advised against them in the general population to be healthy. At the moment, the most intense debate about the coronavirus centers on the role that aerosols (respiratory droplets with a diameter less than 5 microns), a phenomenon also known as “airborne transmission”, have in the contagion and outbreaks of coronavirus .

At the beginning of the pandemic, aerosols were thought to only play a role in virus transmission in very specific settings, such as in the medical setting when certain tests are performed that cause the abundant release of aerosols. This is what various health authorities defended, including the WHO, just a few months ago. However, the scientific evidence on the dispersion of aerosols and its role in the transmission of coronavirus outside hospitals and health centers has been increasing. These are the keys to understanding the current situation of the debate on aerosols in the transmission of coronavirus:

The size of the drops

When people breathe, speak, sneeze, sing or scream, they release multitudes of tiny droplets of extremely different sizes. Droplets with a larger diameter (Flügge droplets), greater than 5-10 microns, do not usually disperse beyond 1 or 2 meters after release. The weight of these droplets limits the time they can be suspended in the air and they settle relatively quickly on floors and surfaces. At present, it is thought that the main way of transmission of the virus is through these droplets, which is why it is recommended to maintain a safe distance and wear masks.

The aerosols, smaller than the Flügge droplets, behave differently. Being lighter, they have the ability to stay in the air for several hours and be carried by air currents for distances much greater than two meters. However, at the beginning of the pandemic few researchers thought that, outside of hospitals, this transmission mechanism could occur in the general population. Aerosols, because of their tiny size, can only contain a very small number of viral particles and, if they are dispersed through the air, they are “diluted”.

As coronavirus infection requires exposure to a minimal amount of virus (which is currently unknown), aerosols were thought to have no ability to infect beyond the safe distance. As more data on infections appeared in multiple parts of the world and it was found that their R0 (to how many people can an infected person transmit the virus) was not especially high, between 1.5 and 3.5, less likely was that aerosols were involved. Viruses as contagious as measles, which have a great capacity to be transmitted by aerosols, have an R0 of between 12 to 18.

Accumulated aerosols in confined spaces

However, in recent months epidemiological studies have been accumulating that showed that infections beyond the safety distance were possible under certain circumstances in everyday situations: closed places, with poor ventilation or air recirculation and with infected people who were for one or several hours in the same room as others. They are common factors that have been observed in outbreaks such as a slaughterhouse in Germany (where a person infected those around him within a radius of eight meters), in choirs, offices, restaurants and a bus, events in which infections occurred that far exceeded the safety distance. What these events suggest is that aerosols could cause infections if they are released and accumulate in the air of a closed space over time.

Given these findings, more than 200 researchers from around 30 countries publicly requested on July 6 to the WHO to rethink its scientific position on aerosol transmission, since this institution claimed that the coronavirus was not transmitted through the air. On July 9, the WHO recognized this possibility albeit with considerable skepticism: “To date the transmission of SARS-CoV-2 by this type of aerosol route has not been demonstrated; much more research is needed given the possible implications of such a route of transmission.” However, the health institution recognized that several outbreaks suggested “the possibility of transmission by aerosols, combined with transmission by droplets, for example, during choir practices, in restaurants or in gymnastics classes” and, in general, in places closed and poorly ventilated.

Indeed, there is no direct evidence of contagion by aerosols. It is also true that there is also no conclusive evidence that the coronavirus can be transmitted by touching surfaces contaminated by coronavirus and that has not prevented the WHO from constantly emphasizing the importance of hand hygiene as a measure against the virus since the beginning of the pandemic. It is not easy at all to demonstrate with direct tests the transmission of the virus by these routes if it is not by purposely exposing people to the virus, something that would be unethical to do.

A matter of probabilities

However, there are more and more indications that reinforce the role of aerosols in infections. A preliminary study (not yet reviewed by other scientists) shows the presence of coronavirus in aerosols almost five meters away from a person infected with coronavirus in the hospital. Also, these coronaviruses had the capacity to infect.

In any case, although these data suggest that aerosols are responsible for infections under certain circumstances, it does not seem that the coronavirus has a special ability to be transmitted in this way, compared to other viruses. It is not a question of whether or not the coronavirus is transmitted by aerosols, but to what extent the coronavirus can be transmitted in this way. The director of the Department of Infectious Diseases at the University of Maryland, Faheem Younus, explained that “If airborne infections (tuberculosis / measles) fly like an eagle, the coronavirus flies like a chicken.”

Efforts are still being made to clarify to what extent the virus, carried by aerosols, can “glide” and be infectious beyond the safe distance. Certain Scientists Believe Scientific Evidence Is Enough to recognize the role of aerosols. However, the WHO is a conservative health institution that, like it did with its position on masks, will only change its discourse when the scientific evidence is solid. In the coming months we will know if, once again, the balance leans towards another change in the recommendations.


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