Close restaurants, bars, hotels, performance venues, sports centers, shops and places of worship… to curb the spread of SARS-CoV-2. Or reopen them to limit the socio-economic impact. But which ones, and under what conditions? Tragic dilemma, which makes political decisions very difficult. Hence the interest of the different models of the spread of the pandemic.
The latest, designed by Californian teams, was published on Tuesday, November 10, in the journal Nature. The innovative approach is based on massive data from the mobile phones of 98 million Americans – made anonymous -, collected between 1er March and 1er may. Hour by hour, their movements have been precisely mapped in ten of the largest American cities (Chicago, New York, San Francisco, etc.). Their movements from their homes to various public places (restaurants, places of worship, shops, etc.) were thus traced. That is, in total, 5.4 billion hours of mobility analyzed, to 553,000 public places and in 57,000 neighborhoods.
The authors entered this geolocation data into a fairly simple mathematical model, which divides the population into four compartments – susceptible (to be infected), exposed, infectious, withdrawn (immune or dead) – and which predicts their evolution of a compartment to another according to precise transmission rules. Then the parameters were adjusted to the actual number of new daily infections.
Restaurants particularly at risk
What does this model show? First, he predicts that a small number of public places, such as full-service restaurants, account for the majority of infections. In Chicago, for example, 10% of public places visited account for 85% of infections.
Importantly, the authors estimated the effects of different reopening strategies. Again, restaurants are particularly at risk. If the city of Chicago had reopened its restaurants to full capacity on 1er May, for example, 600,000 more infections would have been to be deplored by the end of this month – a risk three times higher than that of the second most at risk category: fitness centers.
Among other examples, the study looked at the impact of reopening toy and hobby stores – an obviously crucial question as Christmas approaches. Verdict: if the city of Chicago had reopened all these stores on the 1er May, the number of new infections would have been a hundred times lower than that due to the reopening of all restaurants. But during the period leading up to Christmas? “These toy and leisure stores would then be much more frequented, which would increase the risk”, notes Pierrick Tranouez, research engineer at Laboratoire Litis, at the University of Rouen, and popularizer at CoVprehension.org.
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