May 18, 2022

INFOGRAPHICS. This is what the fourth wave of Covid-19 in France looks like, faster and more unpredictable

On May 10, the Prime Minister was optimistic. “We are finally in the process of getting out of this health crisis in a sustainable way”, declared Jean Castex to Parisian. Difficult to guess, at the time, that a month and a half later, a new epidemic wave of Covid-19 was going to emerge. Ironically, the day after this declaration, a first case of the Delta variant was detected in Hauts-de-France.

This fourth wave is no longer in doubt: this variant initially identified in India has become the majority in France and all the indicators, which had reached particularly low levels in June, have started to rise again. From June 28 for the number of cases, July 8 for hospitalizations, July 14 for intensive care admissions, and six days later for deaths. With more than 20,000 cases per day on average, this wave, steeper than that of the fall, presents several peculiarities.

An unprecedented increase in new cases

On June 26, the incidence rate was close to the low it had seen in the summer of 2020, with 18 new cases per 100,000 inhabitants. But it took just over a month for him to climb back to the worrying level of 224 new cases per day on July 30. What alarm Olivier Véran in the National Assembly. “We have an increase in the circulation of the virus of around 150% over a week: we have never experienced this, nor with the Covid [la souche historique du virus], neither with the English variant, neither with the South African nor with the Brazilian “, declared the Minister of Health to the deputies, on July 20.

In fact, when compared with that of fall 2020, the current wave seems more sudden: it starts lower, but is growing faster than in October 2020, when the incidence started to increase more. slowly. However, for several days, the incidence has increased more slowly, and now seems to stabilize around 250 new cases per 100,000 inhabitants. Well below the peak of the October 2020 wave, located around 500.

Renaud Piarroux, epidemiologist and head of department at the Pitié-Salpêtrière hospital in Paris, confirms this: one of the essential elements explaining this rapid increase is the arrival of the Delta variant. “We went from a virus that was moderately contagious to a virus that is one of the most contagious viruses that we know. This allows it to be transmitted in conditions where it was transmitted shortly before”, he judges. For him, the context has also played a major role in this increase in cases, which is more marked among young people: the lifting of barrier gestures, the reopening of public places and the holiday effect have resulted in a “festive type transmission much more marked than last year”.

Another point raised by the epidemiologist: the geographical distribution of new cases, very heterogeneous, has nothing to do with that of the first or second waves. While last fall, the incidence was higher in Auvergne-Rhône-Alpes, Ile-de-France and Hauts-de-France, the current new cases are concentrated in the southern half of the country, more precisely in the departments bordering the Mediterranean and the Atlantic coast.

More and more hospitalizations, but fewer than last fall

At the hospital too, the indicators are on the rise. New hospitalizations fell from 110 per day in early July to 522 per day on average on August 2. New critical care entries stood at 23 per day on July 8, but are now 114 per day. And this increase should continue for a few more days: even if the number of new cases is increasing less quickly, there is generally a one week lag between the curve of cases and that of hospitalizations. Deaths began to rise again on July 20, from 17 per day to 38 on average in early August.

Compared to last fall, hospitals are less affected by the epidemic wave currently hitting France. At least for the moment. According to our calculations based on data from Public Health France, for 1,000 new infections, there are currently 23 new hospitalizations and 5 intensive care admissions. In mid-October, for 1,000 new cases, there were 48.5 hospital admissions and 8.5 new critical care patients. There are therefore two times fewer hospitalizations.

For Renaud Piarroux, this is the positive effect of vaccination. As of August 2, 53% of French people were fully vaccinated, and 63.4% of them had received at least one dose. A reassuring point, according to him. “In some very touristic areas, we can have hospitals where the situation can get complicated. But elsewhere, this should not go up as high as in other waves, he believes.

Many uncertainties for the coming weeks

However, the situation is highly unpredictable. “We cannot make two month forecasts”, warns Renaud Piarroux. As has been the case since the start of this health crisis, many uncertainties hover over: those linked to the Delta variant, the continuation of the vaccination campaign, the measures that will be taken locally or nationally …

In the UK – as in other countries affected by increases in similar cases, but occurring earlier than in France – the curves have started to reverse, contradicting many of the projections. What plunge some observers into a relative incomprehension. Even in France, some departments have started to decline in the number of cases in recent days. As our epidemic dashboard shows, the Pyrénées-Orientales, Charente-Maritime and the Somme have a falling incidence rate. A recent change, which seems to extend to other departments, but to be observed with caution.

Can we explain it? Asked about this point, Renaud Piarroux leaves a silence, before advancing some ideas: “Measures have been taken in some areas, people are paying more attention, and are increasingly vaccinated.” One thing is certain: according to a study by the Drees (the statistical service of the Ministry of Health), carried out in early July, unvaccinated people accounted for 85% of new hospitalizations.

* Methodology : to define the dates of the two waves and the ‘j zero’ indicated in the graphs, we analyzed the growth rate of the incidence between a day D and a day D-7. The growth rate is the rate at which the curve increases. If it is 100%, the indicator doubles; if it is at 0%, it stagnates, and if it is negative, the indicator decreases. We set the ‘J zero’ on our graphs when the incidence growth rate exceeded 50%: this is when the curve begins to increase further, thus corresponding to the idea of ​​a wave. epidemic. The D0 is thus set for October 10, 2020 for the fall wave and July 8, 2021 for the current wave.