Omicron: wearing a mask outdoors, a measure not understood but useful

It is a step backwards that the Parisians would do well without. From this Friday 6 p.m., the wearing of a mask becomes compulsory again on the public highway for people over 11 years old, with the exception of those traveling by car, bicycle, two-wheelers or practicing a sporting activity. A similar measure, with different contours, however, has been decreed in other Ile-de-France departments and in certain cities such as Lyon, Tours or La Rochelle. For some residents, it is incomprehension. Vaccination has made good progress in the country and contamination is mostly indoors. So why bother to wear the mask as soon as you leave your home?

To answer this question, a little review of the scientific literature is required. A priori, this is rather favorable to skeptics since it clearly shows that the contamination is much less strong outdoors. With two years of hindsight on the epidemic, we also know that some contaminations seeming to come from the outside come in fact from the inside. This is the case, for example, with the cases of Covid among farmers, favored by the accommodation of seasonal workers or clusters that have appeared in ski resorts, caused by festive evenings in bars and hotels.

However, the circulation of covid outdoors remains a reality. We can estimate, by looking at the scientific literature, that it is at the origin of 10% of cases, explains Olivier Damette, Lecturer-researcher at the Office of Theoretical and Applied Economics (BETA) and associate researcher at the “Economics” chair. du climat “from Paris Dauphine University. To take this sizeable fraction into account, the recommendations for the fight against the epidemic have therefore evolved.

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“As we learned more, we realized that at the onset of infection, the transmission of the virus can be increased and many people don’t even have symptoms during this time. at that point we changed the recommendations and started saying that everyone who was indoors or outdoors should wear a mask because by not knowing they were infected they could spread the disease. epidemic”, explains Greg Martin, physicist specializing in infectious diseases in Washington.

Concentration and duration of exposure, the two criteria that count

Of course, it makes no sense to wear the mask while alone in the street. But in the crowded aisles of a Christmas market or in the narrow streets of a city center, the risks of contamination are higher. Especially since the Omicron variant is extremely contagious. “In fact, in terms of contamination, two criteria count according to Olivier Damette: the concentration of virus in the air and the duration of exposure to this virus”. And even if the conditions seem more favorable outside, they cannot rule out contamination. Even in very ventilated places like ski resorts, several scientists told L’Express last winter.

Faced with this permanent variation in risks, wearing a mandatory mask both outdoors and indoors can be a good thing. “It is first and foremost a reminder of barrier gestures after months of slackening. Wearing it everywhere also means thinking about it more often. The more we follow the recommendations, the more effective they are. sees particularly in Asia where the sanitary directives are generally better respected “, estimates Olivier Damette. Wearing a mask outdoors is also less costly, from a political point of view, than confinement.

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We can of course be resistant to it. But when it comes to masks, a few simple behaviors are enough to reduce the risks, say epidemiologists. A recent article of the New York Times evokes for example the rule of “two in three”: when you are outside, you have to make sure you are at a distance Where mask. If, on the other hand, we are indoors, then we must stay as far away as possible and mask. “In the series of habits to take, we could also think about the adoption of the FFP-2 mask in the most frequented places, underlines Olivier Damette. Of course its price is higher, but it holds better on the face and it offers a protection superior to that of surgical masks “. Two criteria that count in Omicron’s time.