Une courbe qui baisse encore plus rapidement qu'elle n'avait grimpé. Le nombre de nouveaux cas de Covid-19 est en chute libre depuis début novembre en France. En moyenne quotidienne sur la semaine écoulée, il est passé de 48 000 le 3 novembre à 27 000 ce 12 novembre, d'après les données de Santé publique France. Cette baisse est encore plus marquée dans certains départements, et notamment à Paris. Dans la capitale, le compteur affichait près de 2000 nouveaux cas par jour le 27 octobre; il s'élevait à moins de 600 ce 12 novembre, là aussi en moyenne hebdomadaire.
"For ten consecutive days, the number of new diagnoses of Covid-19 has decreased," said the Minister of Health, Olivier Véran, on Monday in an interview with the newspapers of the Ebra group.
These changes are also reflected in the famous incidence rate, corresponding to the number of new cases over seven days per 100,000 inhabitants.
So how do you explain them?
Antigenic tests not yet taken into account
It should first of all be noted that these numbers correspond to the new cases identified by RT-PCR test, recorded in the Si-Dep file.
Much faster antigen tests (15 to 30 minutes instead of one or more days for results), which pharmacists and nurses have been allowed to perform since October 26, are not yet taken into account.
A decree authorizing these health professionals to fill out the Si-Dep file themselves has just been published this Sunday in the official journal.
Public Health France was not able to tell us, at the time of publication of this article, if this would appear on Monday evening.
Still, in recent weeks, people have been able to take an antigen test rather than a PCR test.
In the event of a positive result, this could have falsified the indicator for new cases, especially since antigenic tests are intended primarily for symptomatic patients and are not recommended for simple contact cases, said the Regional Health Agency. 'Île-de-France in a strategic document published on November 2.
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What impact can this failure to take into account have had?
In its weekly epidemiological update of Thursday, November 12, Public Health France wanted to be rather reassuring.
"According to the data available on the volumes of antigenic tests carried out, it seems that their exclusion does not call into question the decrease in incidence observed", wrote the health agency.
8,691 positive cases by antigen test were identified from November 2 to 8, against 291,029 by PCR test over the same period.
The ratio is therefore low, but it is more or less important depending on the region and, above all, it should increase further as the antigen tests are implemented.
“There may indeed be an impact linked to the fact that people who would normally have been tested by RT-PCR were tested by antigen test.
But, as we are observing a slowdown in hospitalizations, the downward trend in new cases is very real and this bias does not appear to be major ”, judges epidemiologist Pascal Crepey, teacher-researcher at the School of advanced studies in public health in Rennes.
Curfew and containment
Among the other reasons for this drop in new cases that are put forward, are of course the effects of the various restriction measures.
We can also see that the peak was reached earlier in the departments of Ile-de-France and in the eight metropolises where a 9 p.m. curfew was put in place from October 17.
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In Paris, Lille and Saint-Etienne, in particular, the incidence began to decrease a few days before the entry into force of national confinement, on October 30.
“The positive effect of the curfew, it is proven and bearers of hope.
When it was set up, the only study showing that it could be effective had been carried out in Guyana, with an argument not necessarily transposable to the metropolis ”, rejoices Pascal Crepey.
Many other measures had already been taken since the start of the school year, particularly in the Paris region and in Marseille.
In addition to wearing masks and applying barrier gestures, bars and restaurants had to close after a certain time, as well as certain places open to the public.
“The various measures put in place, in particular the curfew, have very likely had an effect.
This partly explains why the Île-de-France was relatively protected while the wave continued to rise in other departments ”, supports Professor Olivier Bouchaud, head of the infectious diseases department at the Avicenne hospital (Bobigny ).
A beginning of collective immunity?
What if Île-de-France also “benefited” from being very affected during the first wave?
This question refers to the concept of collective immunity, acquired when a sufficient part of the population has been contaminated and therefore contains antibodies that can prevent reinfection.
The basic reproduction number (R0) of SARS-CoV-2, that is to say the average number of people that an infected individual will infect, is estimated at 3. However, collective immunity would be reached when the percentage of the infected population is at least 1-1 / R0.
That is, in the case of this coronavirus, about 66%.
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But this rate could be lower now that a certain number of barrier gestures are respected and that social contact is much more limited.
“This reproduction number R aggregates both the characteristics of the virus but also the context.
It is therefore necessarily impacted by preventive measures, ”explains Pascal Crepey.
If we now base ourselves on an R of less than 3, this results in a collective immunity percentage of less than 66%.
“Do we have to wait until at least 50% of a population is immunized before we can start to control the virus?
Or can we be satisfied with around 25%?
“, Asked himself the President of the Scientific Council, Jean-François Delfraissy, in the Canard enchaîné of November 11.
"Not a specific factor but a set of reasons"
At the end of the first wave, two studies by the Institut Pasteur and then by Inserm estimated that around 10% of the population had already been infected in Île-de-France and in the Grand Est region.
This figure has of course climbed over the course of the second wave, but it is too early to know by what proportions.
Moreover, "it is obvious that it is much higher in certain districts of Seine-Saint-Denis, for example", notes Professor Bouchaud, referring in particular to very dense areas and with poorer sanitary conditions.
“Collective immunity is not black or white.
Even if it is not completely affected, the virus can already block this or that immune person, ”he adds.
And thus break the chain of transmission.
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Other events independent of the epidemic and the measures taken to deal with it, such as school holidays or a certain warming of temperatures, could also have had an impact on the number of new cases.
As Pascal Crepey sums it up, "there is not a specific factor but a set of reasons explaining this decline, and we cannot yet provide a definitive explanation".