Question asked by BARALE on 08/22/2020
The start of the school year is masked, In the enterprises as in classrooms. Before these decisions were made, one could read, in Libé on August 14, a forum of a collective of “health professionals” (“Covid-19: the risk exists in all enclosed places, in classrooms as well as in offices”) : “We affirm it without ambiguity: Sars-CoV-2 is transmitted through the air and does not make the mask compulsory in classrooms or amphitheatres, in open spaces, meeting rooms, workshops and shared offices does not comply with science and WHO data. ”
In the light of these very affirmative lines, you ask us about the state of science about the transmission of the virus in the air. There is in fact no consensus on the circulation of Covid-19, from one individual to another in the general population, via “aerosols”, these microdroplets which remain in suspension and which could be vectors of active virus. and infectious.
Two doctors, authors of the forum, recognize from CheckNews that the wording “unambiguously: Sars-CoV-2 is transmitted through the air ” is not ideal. But that it responds to a precautionary principle: there is a risk that airborne transmission exists, including outside hospitals or laboratories (where it has been documented).
You also asked us Is wearing a mask outside useful?
Warning from scientists
At the beginning of july, 239 scientists from 30 countries warn about the aerosol distribution of the virus. Addressing international health institutions (including the WHO) and national, they recommend great vigilance regarding the transmission of the virus by microdroplets: “Of course, aerosol transmission of Sars-CoV-2 is not yet universally recognized; but, according to our collective assessment, there is sufficient evidence that the precautionary principle should apply. In order to control the pandemic, while waiting for the possibility of vaccination, all transmission routes must be cut off. ”
Several articles or forums have been devoted to the impact of this publication on the authorities (in French, in English). In France, the Director General of Health, Jérôme Salomon, approached the High Council for Public Health. The HCSP had already expressed itself on the subject in a notice published on April 8 (and to a lesser extent in a notice published on April 24).
In this April text, we read in particular: “Aerosol transmission is biologically plausible when (1) infectious aerosols are generated by or from an infectious person, (2) the pathogen remains viable in the environment for a period of time, and (3) tissues targets in which the pathogen initiates infection are accessible by aerosol. […] For Sars-CoV, the level of evidence for aerosol transmission indicates that the evidence for conditions 1 and 3 is moderate and strong for condition 2. “
CheckNews also made a point on the state of science regarding aerosolization at the beginning of April. In an article published in May on the circulation (or not) of the new coronavirus in air conditioning, we also wrote: “Although it is possible for smaller virus particles to remain suspended in the air (this is called an aerosol), an infection requires that a sufficiently large number of active copies of the virus enter the body: this is the infectious dose. In the vast majority of situations, wind, drafts and ventilation reduce the concentration of viruses in the air, which significantly lowers the risk of infection. On the other hand, a greater risk seems to exist in intensive care situations, in premises where caregivers remove their protection and in hospital toilets. “
Opinion of the High Council for Public Health
After a request from the DGS, the HCSP therefore produced a new opinion, dated July 23: “Sars-CoV-2: updating of knowledge on the transmission of the virus by aerosols.” Conclusion ? “The HCSP considers that after updating the data in the literature, airborne transmission of the Sars-CoV-2 virus should be considered in enclosed spaces, in particular poorly ventilated and insufficiently ventilated, and in outdoor gatherings.” And to urge the public to wear a mask “In all enclosed public and private collective places as well as in the event of a gathering with a high density of people outdoors in order to limit the emission of respiratory particles.”
More specifically, the High Council cites several scientific studies published in recent weeks on the subject. One, published in early April in Nature concludes that “Aerosol transmission is a potential mode of transmission of coronaviruses, such as influenza and rhinoviruses.” The results ofanother, published in June 2020 from laboratory experiments, suggest that the“Infectivity and virionic integrity [du Sars-CoV-2] can be kept for up to 16 hours in aerosols of a breathable size ”.
The HCSP also refers to a literature review produced by the Canadian National Collaborating Center for Environmental Health (funded by the Public Health Agency of Canada) regarding “Risks and precautions associated with Covid-19” as part of choral singing. The CCNSEA notes in particular that “More and more, it is believed that transmission by small droplets or by aerosols produced during speaking, singing, shouting or breathing is a major route of contagion. […] Aerosol transmission could occur in contexts where particles accumulate in closed, unventilated rooms where many people are gathered for a long period ”. In the opinion of the HCSP, some case studies follow which have “Describes the occurrence of contamination by the Sars-CoV-2 virus of people gathered in confined spaces”.
In the laboratory, in the hospital, in general
In this opinion, the HCSP therefore identifies studies that point in the same direction: airborne transmission of Sars-CoV-2 is possible. But it is not possible in the same proportions, depending on the situation.
Thus, the High Council writes “take into account” two elements. First, the open letter from scientists mentioned at the beginning of the article. Didier Lepelletier, co-chair of the HCSP Covid-19 permanent working group, points out that this letter was written by physicists, “Who believe that the dichotomy between droplet and microdroplet [aérosol] is irrelevant, for them everything is aerosol ”. What’s more, continues the head of the Bacteriology – Hospital Hygiene Department of the Nantes University Hospital: these physicists “Are based on models, for example in the laboratory, which for some have suggested that the virus could circulate in the form of aerosols”.
Second element considered by the HCSP for its opinion this summer: a position statement from the beginning of July from the WHO. It is fully readable (and in English), or in summary form (in English), tells us the World Health Organization.
“Is the virus spread by aerosols?” wonders the WHO, which distinguishes two cases. On the one hand, in a healthcare setting: “Some medical procedures can produce very small droplets that can stay airborne for longer periods. [par exemple une intubation ou une extubation]. When such medical procedures are performed on people infected with Covid-19 in healthcare facilities, these aerosols may contain the Covid-19 virus. These aerosols can potentially be inhaled by other people if they are not wearing the appropriate personal protective equipment. ”
Another scenario, in the general population (excluding hospitals): “Cases of Covid-19 have been reported in some closed places, such as restaurants, nightclubs, places of worship or workplaces where people can shout, talk or sing. In these cases, aerosol transmission, especially in those closed places where there are overcrowded and insufficiently ventilated spaces where infected people spend long periods with other people, cannot be excluded. ” And the WHO to insist (like the HCSP): “Further studies are urgently needed to study these cases and assess their importance for the transmission of Covid-19.”
Less contagious than chickenpox
“There is never only one mode of transmission of a virus, recalls Didier Lepelletier, but there is always a preferential mode. “ It can be hand-carried, or aerial for example. And among the modes of aerial transmission, there are therefore, schematically, droplets and aerosols. In the current state of knowledge (and for several weeks), “Droplets are a preferential mode of transmission over aerosols”, observes the member of the HCSP.
Several scientists contacted believe that if aerosols were the main vector of Sars-CoV-2, the R0 (initial reproduction number) would be much higher. As it is, for example, for pulmonary tuberculosis or chickenpox (which are transmitted by aerosol). Whose R0 fluctuates around 10 (much more than for Covid-19).
In a recent study published in the British Medical Journal, the authors find that the risk that a person without symptoms (but who can, for example, emit aerosols just by breathing or talking) contaminates those around them depends on the density of people, their activity and ventilation. This is in line with what the High Council expressed in its opinion of 23 July, according to which “Three favorable conditions are to be remembered concerning the transmission of the virus: the conditions of ventilation and air flow; atmospheric conditions (low temperature, humidity); physical activities and efforts practiced in confined spaces ”.
It should be noted that neither the HCSP, nor the WHO (nor the signatories of the forum cited above) explicitly recommend wearing a mask outside of these situations (for example in general, in the street). Still, specialists in these subjects would like to see studies develop to better understand the modes of transmission of this virus.
In summary : aerosols do not appear to be the preferred mode of transmission on Sars-CoV-2. However, certain observations in hospitals or in laboratories suggest that there may be airborne transmission, without there being a consensus on the risk or the probability of it occurring. In the general population, there is no proof or scientific measurement of the risk of airborne transmission, but this risk is probable according to the scientific community, and it is all the more important in poorly ventilated environments or outdoors in the event of large gatherings. Hence an incentive, under these conditions, to wear the mask in a closed environment, under the precautionary principle.