what is the real risk of contamination?


  • In Paris, organizing an event with 50 people poses the risk that at least one person is 86% infected; in the Rhône, it is 99%.
  • Just over a month after Canadian Thanksgiving, the daily number of cases has nearly doubled.

Before making plans for Christmas or New Year’s Eve, it is possible to calculate the risks. Researchers at the University of Georgia Tech in the United States have developed a site which makes it possible to estimate the risk according to the official number of new cases reported each day in a given place. Launched in July, the site saw its method validated by the publication of an article on November 9 in the prestigious scientific journal Nature Humlan Behavior.

At 50 guests, the risk of a person being infected exceeds 50% everywhere in Europe

For the time being, it is impossible to predict which restrictive measures will be put in place to regulate the end of year celebrations. Thanks to the site developed by American researchers, it is possible to estimate in real time the risks incurred if these Festivals were to take place now and that we would be authorized to gather. To calculate these risks, the model takes into account, in addition to real-time data on the Covid-19 pandemic, the fact that the actual number of contaminations is 5 to 10 times greater than the number of positive tests. The user thus has the choice of calculating the risk according to these two hypotheses, 5 or 10. Then, we can select the number of guests that we plan to meet or join: 10, 25, 50, 100 , up to 5,000 people. In addition, the site estimates, as a precaution, that an infected person remains positive for 10 days while the duration of contagiousness is estimated between 5 and 6 days.

With these data, organizing an event bringing together 10 people in Paris has a 16% chance that a person will be infected with Covid-19. The figure rises to 45% in the Savoie department and stands at 27% in the Nord department. If the number of guests increases, the percentage goes up. If we push 50 people, we exceed 50% probability of being in the presence of an infected person, in Europe and the United States. In Paris, organizing an event with so many people poses the risk that at least one person is 86% infected. In the Rhône, it is 99%.

Canadian Thanksgiving, the bad example

Another example that can allow us to anticipate the consequences of regroupings during the end-of-year celebrations is Canada. On October 12, Canadians celebrated Thanksgiving. Two weeks after this national holiday, the number of daily cases recorded in the country increased from 2,300 on the eve of this holiday to 3,000. Today, 4,000 cases are recorded every day. “In some territories, we know the gatherings over Thanksgiving weekend contributed to the increase in the number of cases we are seeing today”, Admitted at the end of October the deputy director of public health of Canada, Howard Njoo. “I think it’s reasonable to think that the share of the increase we’ve seen in Toronto is tied to Thanksgiving”, Continued Eileen de Villa, head of health for the city.

In the United States, Thanksgiving takes place on November 26 and is raising fears of a surge in cases. Prevention messages are multiplying, in particular by the Centers for Disease Control and Prevention (CDC), health authorities, who recommend celebrating the party outside or failing that with the windows open and respecting barrier gestures. President-elect Joe Biden urged Americans not to regroup with more than 10. Not sure whether to be understood as a poll in Ohio state suggests that two in five residents plan to move out. group together more than 10 around the traditional turkey.

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