Covid cases in France are no longer falling, but don’t panic

SCIENCE – As the whole world looks to Ukraine and Russia, is the coronavirus playing us a bad game? In recent days, the drastic decline of the fifth wave has stopped. Worse, the number of positive cases even seems to be increasing. Even though the end of the vaccination pass and the inner mask is planned in a week.

At the moment, however, there is no need to panic. Firstly, because this recovery, which could partly be due to the start of the school year, is still light and uncertain. Then, because there is a good chance that the upcoming increase will be small.

Simon Cauchemez, modeler for the Pasteur Institute, even admits to us that he is “quite optimistic for the coming weeks”. Well on one condition.

An increase in sectorized cases

Above all, it should be borne in mind that the situation is not the same everywhere in France. In less than ten departments, the incidence increases in a week. In general, if we look at the map of metropolitan France, there seems to be a trend: the end of the decline in the number of cases is more pronounced in regions where the school holidays ended earlier.

This distinction is all the more striking if we compare the average incidence in the departments according to holiday zones. The start of the school year took place on February 21 in zone B, on February 28 in zone A and on March 7 in zone C.

As can be seen in the graph below, the incidence in zone B is increasing among young people under 18 years of age, especially among the youngest (usually unvaccinated). In zone A a plateau is formed, while in zone C the number of cases continues to fall.

“It’s not surprising that the holidays have an impact” on the circulation of the virus, Simon Cauchemez recalled. But as the incidence increases, so does the number of tests among the youngest, as can be seen in the graph below.

Could this increase in incidence be partly due to a simple change in the screening system?

“It’s always difficult to fully characterize how detection rates change over time, but looking back, when dynamics changed, it wasn’t just due to a change in systems screening,” specifies the modeler.

Cases are rising, but moderate

Especially since this epidemic recovery fits pretty well with the latest models from the Institut Pasteur, published on February 21. “The aim was to look at the impact of the Omicron subvariant, BA.2, as well as that of a behavior change,” explains Simon Cauchemez.

According to the considered scenarios, BA.2 alone could not cause an epidemic revival because its transmission advantage is too small compared to the vaccination coverage and immunity obtained by the huge wave caused by Omicron.

On the other hand, if the French were to change their behavior, the model predicted a plateau or even a revival of the Covid-19 epidemic. “We assumed an easing in early March, but the current plateau is coming earlier than expected. If we let the model stick to reality, it suggests that there has been a relaxation in behavior from February 22, “says the researcher.

The good news in all this is that even with a significant increase in our high-risk contacts and thus in the transmission of the virus (+50%), the model predicts a sixth wave as opposed to the previous one, as we can see from the graph below , which takes into account a change in behavior on February 22.

Duration of immunity, a great unknown

The reason? Again, vaccination and the fifth wave mean there are no longer enough people likely to be infected with the coronavirus for the epidemic to kick off at a gallop.

Of course the models are not perfect. They may be wrong if the assumptions made to imagine future scenarios are not correct. And if Simon Cauchemez wants to be optimistic in the coming weeks, there is for him a rather optimistic hypothesis that remains uncertain: the duration of the immunity granted by Omicron.

“We were of the opinion that the immunity we acquired after an infection does not drop until April 1, when our simulations stop. But that’s a pretty strong assumption that could make our projections too optimistic,” he warns. “Therefore, we will stop our forecasts on April 1 and say that this assumption is acceptable for the very short term, but not beyond.”

The Institut Pasteur is already working on perfecting its model, taking into account the duration of immunity. “In a few weeks we should be able to run scenarios for a more distant horizon, until next winter.”

In the meantime, we can only hope that the current model was right. And to remember that even in the case of a relatively large wave, the virulence of Omicron in combination with our level of immunity should reduce the proportion of severe forms compared to the first waves of the pandemic.

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