Since the beginning of the COVID-19 pandemic and its very rapid spread, many scientists around the world have been studying how the coronavirus is transmitted between people. Health authorities, WHO in particular, have so far estimated that the virus is transmitted mainly through droplets emitted by coughing or sneezing - which then contaminate other people, either directly by reaching them (airborne transmission route) or through objects on which the droplets have fallen and which others will touch.
But there is a lively debate about this: 239 scientists from 32 countries have just jointly published an open letter to WHO, claiming that the coronavirus is well airborne and that its main route of transmission is in the form of micro-particles of water carrying the virus, floating and diffusing in the air for, potentially, several hours. This view on the mode of spread of the virus is crucial for the barrier measures to be implemented to prevent the spread of the virus. In this article, we review the information that has been disseminated in recent weeks and months about the transmission of COVID-19 by air.
What is the difference between large droplets and micro-droplets (aerosols)?
When we talk, we project invisible micro-droplets of saliva otherwise known as aerosols. These aerosols are microscopic in size, allowing them to be suspended in the air for long hours. Aerosols may contain viral particles. On the other hand, the large droplets that we emit when we sneeze or cough, due to gravity, will fall to the ground more quickly or be deposited on objects.
What are the indications of transmission of the virus in the air by these aerosols today?
WHO wrote in a scientific note on March 27 that evidence of viral RNA "is not indicative of a viable virus that could be transmissible". However, some scientists say that little evidence has been put forward to support the assessment and that the lack of evidence does not mean that SARS-CoV-2 is not transmissible through air. 239 scientists in 32 countries described evidence in late June that smaller particles can infect people and asked WHO to revise its recommendations. This information is reported in all media around the world. According to Dr Marr, an expert on airborne transmission of viruses at Virginia Tech, "the focus has been on handwashing and social distancing, but if the WHO recognizes that aerosol transmission is occurring, we may have additional advice. "In the minds of the scientists working on this topic, there is no doubt that the virus is airborne," says Lidia Morawska, an aerosol specialist at Queensland University of Technology in Brisbane, Australia.
Dr. Lindsey Marr, a U.S. scientist recognized as the leading expert on airborne spread of viruses, has studied and demonstrated the spread of the flu virus, which is carried by micro-particles of water in the air and can remain there for an hour or more. She agrees with the other scientists who signed the open letter to the WHO that the airborne route plays an important role in the spread of COVID-19.
According to a study published by the PNAS last May, airborne transmission is believed to be a major contributor to the increase in infection before the introduction of mandatory mask use in many countries.
In a closed environment, virus-carrying aerosols accumulate easily, and these high levels facilitate transmission. In many countries around the world, social distancing and containment measures, combined with hand disinfection, were implemented at the beginning of the pandemic (Case A in the diagram above). These measures minimized transmission through short-distance contact but did not prevent long-distance transmission by air. Mandatory face protection measures, on the other hand, were effective in preventing airborne transmission. Wearing a mask blocks spraying and inhalation of aerosols and contact transmission by stopping the excretion of viral droplets (case C in the diagram above).
Fine particles in the air can act as a carrier for the virus to remain in suspension.The physicist and chemist Jean-François Doussin explains that the concentration of virus-laden fine particles would represent a very small part of transmission. Others, however, such as Isabella Annesi-Maesano, head of the Pierre-Louis Institute of Epidemiology and Public Health, present arguments suggesting that airborne particles of pollution would favour the transmission of the disease. However, all agree that pollution irritates the mucous membranes of the respiratory tract and lungs, making it easier for viruses to penetrate, and for fine particles to carry viruses.
Air pollution, especially high concentrations of nitrogen dioxide, can be particularly important in the context of respiratory tract infections. An observational analysis during the DESK3 outbreak of severe acute respiratory syndrome (SARS) in China revealed that mortality rates were higher in some urban areas. These areas had high levels of air pollution, although these results were not adjusted for important confounding factors such as age, gender and various chronic diseases.
A study conducted in November 2020 in the United States states that a slight increase in long-term exposure to PM2.5 would lead to a sharp increase in the mortality rate related to VIDOC-19*. * Wu, X., Nethery, RC, Sabath, MB, Braun, D. and Dominici, F., 2020. Air pollution and mortality by COVID-19 in the United States: Strengths and limitations of ecological regression analysis. Science Advances, 6 (45), p.eabd4049.
