Covid-19: Do outdoor gatherings promote contamination?

“We get up immediately”, “jI’d rather stop thieves than do kindergarten with youA few minutes before the 6 p.m. curfew last Sunday, Parisian police evacuated onlookers who were taking advantage of the rays of the sun on the banks of the Seine, even using boats to broadcast audio messages urging everyone to return. at home. An hour earlier, the police filtered or even prohibited access to the docks, as several journalists observed on the spot. A somewhat infantilizing and widely publicized operation, the day after Prime Minister Jean Castex’s request to strengthen police controls, while the epidemic situation remains worrying – around 22,000 cases and 300 deaths per day on average over the past seven days, more than 25,000 hospitalizations in progress – and that the weather, lenient, favors external appointments.

But what does science say about these gatherings? Do they represent a health risk? Doesn’t the wind help disperse the virus? Isn’t it better to stay outside, especially if the temperatures allow it, rather than to be confined inside? And can we believe that we are safe outside?

“Aerosol contamination is unlikely outdoors”

There are three known modes of Covid-19 contamination. The first is related to aerosols, these tiny droplets emitted when a person breathes, speaks, sings, screams or even coughs and sneezes. They carry only a small dose of the Sars-CoV-2 coronavirus but can, on the other hand, float in the atmosphere for several minutes or even hours before falling to the ground. If they accumulate in the ambient air, typically in closed and poorly ventilated spaces, the cumulative viral load of the droplets can then cause infections if the people who breathe them do not wear a mask, or poor quality masks.

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The second mode of transmission, the “direct route”, is caused by the large droplets also emitted when a person sneezes, coughs, or even sputters while singing or shouting. They carry larger amounts of the virus, and can infect a person through the mucous membranes of the eyes, nostrils or mouth. But their weight causes them to drop quickly to the ground, preventing them from accumulating in the air. “It has also been shown that when people speak, and in particular when they pronounce certain consonants such as T and D, they can emit large droplets”, underlines Antoine Flahault, epidemiologist and director of the Institute of Global Health at the Faculty of Medicine of the University of Geneva.

The last mode of transmission involves surfaces contaminated with large droplets. Theoretically, if a person sneezes on a table, or coughs into their hands and then grabs a doorknob, then a healthy person touches that surface before putting their hand to their face, they can be contaminated. .

“For the aerosols emitted by a person with Covid-19 to contaminate someone else, you need a significant viral load and a sufficiently long exposure time, and aerosols are very light and dilute quickly in the body. air, recalls Antoine Flahault. Contamination by aerosols therefore occurs mainly inside buildings, in particular in poorly ventilated places, and is unlikely outdoors, at least not more than passive smoking outdoors. ” The specialist also recalls that the “ballistics” of the large droplets, the second mode of transmission, shows that they fall to the ground after approximately 2 meters. A contaminated person who sputters, sneezes or coughs within two meters therefore represents a potential risk, especially if they are not wearing a mask, both indoors and outdoors. As for contamination by surfaces, if WHO first recommended cleaning potentially contaminated surfaces, many experts subsequently noted that the risk of contamination by infected objects is “not significant, or even not measurable“, and a WHO expert even said that there is”little evidence of transmission from infected objects, even if this remains a mode of transmission considered to be possible. “” And outside, the ultraviolet radiation from the sun strongly degrades the virus, just as the ambient air tends to dry it out, which is why it does not There is no proven risk of contamination via contaminated surfaces outside, ”says the director of the Institute for Global Health.

No externally documented cluster

The contamination on the outside is therefore rare, but not impossible, especially if the barrier gestures – wearing a mask and social distancing of 2 meters – are not applied. The risks increase in spaces where the crowd is very concentrated and where there is little air circulation. The circumstances are therefore important, and justify the wearing of the mask outside, at least as a precautionary principle and in particular near symptomatic people or when the human density is high. “But to date, there is no evidence of major sources of contamination outdoors, sums up Antoine Flahault. Neither for the Fête de la Musique, nor for major events such as those of Black Lives Matter, etc. “Even the rave party on December 31, organized in a hangar open in Brittany, widely criticized by the authorities, does not appear to have resulted in a cluster. If France’s retrospective tracing capacities are limited – the ARS was able to consult only 29 tests out of the 2,500 participants, all negative – no explosion of the epidemic was observed in Brittany. in the following weeks.

“But several Asian countries or even Australia – which are used to tracing all the chains of contamination, even going so far as to carry out genetic sequencing of positive people to understand by whom they were infected and where the clusters – have carried out numerous studies which demonstrate that there are no traces of major sources of contamination outside, ”insists the epidemiologist. After discovering ten cases positive in their city, the authorities of Melbourne (Australia), which hosted the US Open tournament in Australia, for example decided to confine the city. While nearly 30,000 people attended the tennis matches, often without wearing a mask, the retrospective investigation made it possible to determine that the contaminations had their origins in a closed place: a hotel.

But why are there so few major sources of contamination outside? “These low cases are probably explained because the aerosols inside constitute the main route of transmission of Covid-19, suggests the Geneva specialist. It is in any case the only one which is really well documented scientifically. The route” ballistic’ [par grosses gouttelettes] is possible, but if there are so few documented cases externally, it is probably because its role is weaker, although it should be remembered that it can have an impact when people are close . ”

Why impose the return to the interior?

Should we then conclude that it is less risky to authorize outside exits after 6 p.m., rather than sending citizens back to their homes or to relatives, where they will not necessarily wear a mask? The political decision of the curfew may raise questions, but we must also not forget that the various measures put in place since the start of the pandemic, including the curfew which leads to the closure of businesses, make it possible to reduce social interactions. avoiding that people from different homes meet and therefore break the chains of transmission.

But rather than employing coercive measures, shouldn’t we rather insist on the danger of meeting indoors rather than outdoors, while easing the pressure on frequentation of outdoor places after 6 p.m. while leaving the interior spaces closed? “Doing more ‘promotion from the outside’ seems to me in any case a good strategy to fight against the epidemic,” notes Antoine Flahault. The mayor of Paris Anne Hidalgo has also suggested, Monday during a press point, that as the good weather returned, school classes could be organized outside. In this context, parks and gardens could therefore be made available to pupils and their teachers. If the proposal risks being anecdotal in Paris – which has 650 schools and around ten times as many classes for few green spaces – this solution could be implemented in many schools in France, or even, why not, in companies.


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