COVID-19 and aerosol transmission, some thoughts

From Marc G. Wathelet, a distinguished virologist who led a team studying the SARS virus

COVID-19 and aerosol transmission, some thoughts


The isolation measures to fight transmission of COVID-19 proposed today in Belgium recommend a distance of at least one meter between individuals. This recommendation is based on transmission of the virus by contact with contaminated material, or, when two people interact face to face, by the very small droplets generated during speaking, coughing and sneezing. Under the influence of gravity, these droplets quickly fall to the ground, within one meter.


However, there is a list of evidence that converges to indicate that COVID-19 is also transmitted by aerosol and when aerosol transmission occurs, it is imperative to have much stricter isolation measures.


An aerosol, literally a solution in air, consists of micro-droplets, which are so small that they stay suspended in the air or fall much slower than small droplets. They are produced during normal breathing and this production is accelerated by speaking or even more by singing, shouting.


The danger of aerosols is thatthey can spread the virus over greater distances and between people who do not interact face to face. Even more stringent control measures are therefore needed for viruses transmitted by aerosol.


While the conventional view is that the SARS-CoV-2 virus responsible for the COVID-19 disease is not transmitted by aerosol, it seems clear to me that this view does not take into account the following observations.


1) It should be noted first that most respiratory viruses are transmitted by aerosol.


2) SARS-CoV and MERS-CoV are uncommon among respiratory viruses, as there was very little evidence that they could be transmitted by aerosol, except during the few super-spreading events. For example, on one flight from Hong Kong to Beijing, 22 people were infected with SARS-CoV from one index patient in 2003, and the distribution of the cases represented on the seat plan for the plane favored the aerosol transmission interpretation.


3) More than 3,000 medical personnel were infected by the new coronavirus in the Hubei province this year. All the images from Wuhan show the personnel with simple isolation masks, not type N95 or FFP2 “duck” masks. The difference between the two types? Isolation masks only protect against expectorated small droplets, not aerosols. 3,000 cases of infection for medical personnel aware of working with an epidemic respiratory virus is very strong evidence of aerosol transmission.


4) In the age of ubiquitous videos, the Chinese government has documented two incidents of transmission in open markets, where an individual was responsible for several infections. In one of the markets, the shortest interaction leading to transmission lasted only 15 seconds and the two individuals were separated by a fruit and vegetable display, at a distance of more than two meters. In the other market, the shortest interaction only lasted 50 seconds. The Belgian government’s position heard on the radio that it takes 4 hours of close proximity in the classroom setting to consider a possible transmission certainly does not take into account the evidence.


5) A group of Japanese epidemiologists calculates that one transmission in two is asymptomatic. Asymptomatic transmission, where the individual does not cough by definition, in such proportion is a clear indicator of aerosol transmission.


6) The difference in the clinical presentation of cases during the SARS and MERS epidemics, and that of COVID-19 cases. There are more cases where the first symptoms are pulmonary with the new coronavirus. Respiratory infections that are caused by small droplets or by contaminated surface begin in the upper respiratory mucosa because they cannot reach the lungs, first hitting another surface in the nose, mouth or throat, and this first infection can then descend into the lungs. The presence of more cases where the first symptoms are pulmonary for COVID-19 is a clear indication that transmission also occurs by aerosol.


7) Super-spreading events: when a single person in Daegu in South Korea can infect at least 37 others in a single episode, it is clear from the description of the event that aerosol transmission was involved.


8) Finally, the high base reproduction number of COVID-19 (~ 7) determined by three distinct groups of epidemiologists, in other words the fact that the virus diffuses with great ease, strongly suggests a transmission that also includes the aerosol mode.


When we consider these eight points as a whole, we come to the idea that aerosol transmission is an important vector for the spread of COVID-19. Therefore, isolation measures that do not take into account aerosol transmission are doomed to failure.

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