The French have seen it all. Deemed unnecessary, then optional, indispensable and finally compulsory. After seven months of pandemic, what does science say about the effectiveness of masks against the spread of Covid-19? If we know that the surgical models and FFP2 clearly reduce transmission in healthcare establishments, “we have little very solid data on their version for the general public”, admits Bruno Grandbastien, president of the French Hospital Hygiene Society ( SF2H). But we are in a situation where we can also reason with common sense and benchmarks from reliable observations. “
The changing position of researchers
The researchers’ position evolved as they discovered the new coronavirus. “At the end of April, continues Bruno Grandbastien, a first publication showed that it probably had the capacity to be transmissible before the first clinical signs of the disease.” Based on the SARS experience in Hong Kong in 2003, learned societies began to recommend covering the face. The idea that this avoids contaminating his neighborhood has spread. That according to which the mask also protects its wearer – such as caregivers in the face of flu patients – has returned to the fore. A scientific consensus has emerged in favor of its use, recommended by the major public health agencies.
What is the evidence? According to a preliminary study published in early August (not yet peer reviewed) and cited by Nature, the weekly increases in per capita mortality are four times lower than elsewhere in regions where masks are the norm or recommended by authorities. In late July, a study co-signed by University of California infectious disease specialist Monica Gandhi suggested that masks, by limiting the dose of virus a wearer receives, could also reduce the severity of infection and inflammatory reaction in case of contamination.
The mask is not enough
To determine their effectiveness, we must go back the three main modes of spread of Sars-CoV-2 : large droplets of more than 10 microns, projected when speaking loudly, coughing or singing; small droplets or aerosols, these clouds emitted by breathing and clearly visible in cold weather; finally, the flat surfaces, keyboards or handles that one touches. Intensive hand washing prevents about 15% of transmissions, says epidemiologist Antoine Flahault, director of the Institute for Global Health in Geneva, citing a meta-analysis of 25 randomized trials. “Large droplets fall quickly to the ground within a radius of 1.5 meters. Unless someone sneezes within 1 meter or land in your eye, nostril or mouth, the likelihood of transmission. is low, continues the professor. At least 70% of contamination is therefore caused by aerosols. “
At least 70% of contamination is therefore caused by aerosols
To visualize their diffusion, Antoine Flahault spins the metaphor of the smoker’s cloud, which floats in the air for a certain time and spreads in an unventilated room. The challenge is to filter these particles smaller than 10 microns. “Research from the National Agency for the Safety of Medicines and the Directorate-General for Armament concluded that type 1 and 2 reusable washable masks filter 90% of particles of about 3 microns, against 95% for surgical “, explains Bruno Grandbastien. Other studies show a small difference in efficacy.
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However, it is difficult to assess the impact of this single fabric barrier in real life. Taking up the image of cigarettes, Antoine Flahault explains that the mask is not enough: “You will be less embarrassed if the room is ventilated, if you are far from the smoker and if you do not stay long. It is the same for reduce the risk of transmission. ” Large randomized controlled trial should lead to conclusion of study, says Nature. Christine Benn’s team, a global health researcher at the University of Southern Denmark in Copenhagen, will complete the recruitment of its 40,000 participants in Guinea at the end of November. Everyone will receive advice on prevention; half of the families will receive bilayer fabric masks. A follow-up of several months will compare their rate of use with that of Covid-19 infections.