Fighting commotion.

Brussels urged, on Thursday, September 24, European countries to tighten their control measures "immediately" in the face of the spread of Covid-19, in order to avoid a second wave of the pandemic.

Olivier Véran, the French Minister of Health, had not waited for this call since he had announced, the day before, new restrictions to control the rebound of contaminations on the national territory.

If the race against time and Sars-Cov-2 is well underway on the Old Continent, a team of scientists has set out to model the future trajectory of the spread of the virus in Europe.

Their simulations, published Wednesday, September 23 in the journal Scientific reports of Nature, predict that all European countries will have reached the peak of contaminations in the current cycle by January 2021 at the latest.

A model borrowed from particle physics

In France, we should not wait too long since this peak would be reached in early October.

In the United Kingdom, the number of new infections would continue to increase until mid-November.

Finally, Poland and Sweden, which rely on collective immunity to fight the virus, should wait until early next year. 

To achieve these results, the team of scientists, led by CNRS physicist Giacomo Cacciapaglia, adopted a novel approach to simulate the evolution of the epidemic over time: particle physics.

The researchers applied an equation used to predict the interactions between these tiny physical elements to the trajectory of Sars-cov-2.

“We had noticed that by applying this model, we obtained a result in line with what happened during the first wave and we therefore wanted to test it to try to anticipate what could happen”, explains Giacomo Cacciapaglia, contacted by France 24.

The advantage of this method lies in its simplicity.

Compared to the mathematical models traditionally used in epidemiology, “there are far fewer parameters to integrate into the equation to perform the simulations”, underlines the physicist from the CNRS.

In this case, the researchers only retained the total number of Covid-19 infections in each country and movements within a territory and between European states from March to July 2020. No need to bother. to take into account factors such as social distancing, the average number of people per household or other criteria that may be found in some models.

It is this simplicity that has made it possible to construct projections at European level.

Indeed, “the more parameters we integrate, the more variations are possible in each territory, which makes large-scale models difficult”, specifies Giacomo Cacciapaglia.

Finland and Italy are doing better than in the model

Other side of the coin: this model, because of its simplicity, only allows "to control a single aspect of the epidemic, that is to say the speed of diffusion of the virus", wishes to underline the researcher.

It does not give any indication of the extent of the epidemic [that is to say the precise number of cases, Editor's note] or the death rate ”.

These scientists also assumed that the same measures used during the first wave are applied to contain the rebound in infections (social distancing, assembly limit, quarantine etc.).

Despite this apparent simplicity, the model has already begun to demonstrate its relevance.

The team's projections, which got down to work in June, already concerned the summer period and turned out to be “consistent, broadly” with the reality on the ground.

Some countries have fared even better so far than predicted by the model.

In Finland and Italy, for example, the virus seems, for now, to have gained ground less quickly than expected.

“One possible explanation is that the measures decided by the authorities [since this summer, editor's note] were more effective than what had been done during the first wave, and the population was also more cautious”, judge Giacomo Cacciapaglia.

In other cases, such as in France, “the virus is spreading slightly faster than what we expected from our calculations”, notes the researcher.

Initial observations which confirm the importance of social distancing and border control measures in the fight against the spread of Covid-19.

For Giacomo Cacciapaglia, the model - if its relevance were to be confirmed - could also prove useful in the future in order to anticipate other possible rebounds or new wave.

But it's also the very type of modeling that we hope we won't have to come out of the drawers too often. 

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