Among the indicators of the Covid-19 epidemic, it is certainly one of the most complex to calculate ... and yet used by several elected officials in their communication.
The theoretical definition of the reproduction number “R” is however simple: it is an “estimate (over the last 7 days) of the average number of people infected with a carrier of the virus”, indicates the Ministry of Health.
Warning: do not speak of “R0”, which is the initial reproduction number (before the epidemic and the measures taken to deal with it).
In reality, there is not one R, but… several.
It all depends on how you calculate it.
It can be based on the numbers of confirmed cases in France (Si-Dep), from emergency room data (Oscour), or from hospitalization data (Si-Vic).
For the first time, Public Health France gave this week an estimate based on hospitalizations.
Coronavirus: the "R", number of reproduction, key indicator ... but volatile
Whatever the basic indicator, the R is calculated using the scientific method known as "Cori", named after Anne Cori, co-author of a study on this subject published in 2013. Several parameters are taken into account, in particular "The average time to the appearance of a new case contaminated by a person carrying the virus", details the Ministry of Health.
The epidemic regresses when the R is below 1, regardless of the method used to calculate it.
"Additional information on the dynamics of the epidemic"
So what is the point of having several?
Public Health France explains that the three reproduction numbers "give additional information on the dynamics of the epidemic".
The first, based on the new cases, "is that which makes it possible to follow the dynamics of the epidemic with the most reactivity".
Except that with the lengthening of the deadlines for obtaining the results, it is no longer very relevant to analyze in real time.
The other two, based on emergency room visits and hospitalizations, “show trends later than new contaminations but are based on data less impacted by the increase in consolidation times”.
They are thus "those to be followed as a priority in the coming weeks", again indicates Public Health France in its last epidemiological point, dated September 24.
With that in mind, let's look at the numbers.
These two Rs that are the most relevant to analyze are, on September 19, above 1.2 in metropolitan France.
After a rebound from the middle of June, the curves seem to start declining for several days.
“They are a little above 1 but they are going down.
If we still make some efforts and if we adopt the right measures where necessary, we can lower them below 1 and thus reduce the circulation of the virus, ”said Olivier Véran during his press conference, September 23.
And we are still much lower than the levels reached during the first wave (above 3).
An indicator to cross with others
There are of course variations depending on the region.
As of September 19, in six of them (Auvergne Rhône-Alpes, Center-Val-de-Loire, Ile-de-France, New Aquitaine, Pays de la Loire and Provence-Alpes-Côte d'Azur), two R's are "simultaneously significantly greater than 1".
These regions "combine an increase in emergency room visits for suspected Covid-19 and an increase in hospitalizations for Covid-19", decrypts Public Health France.
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This reproduction number is now one of the weapons used by several Marseille elected officials in the battle they have engaged with the government to denounce the strong restrictions that come into force this weekend.
The mayor of Marseille, Michèle Rubirola, assured France Inter on Thursday that he had passed "below 1" in Marseille.
Our territory has an R of less than 1, ie one of the lowest contamination rates in France, ”his first deputy, Benoit Payan, also exclaimed on Twitter.
These elected officials therefore seem to evoke the R based on the new cases.
The famous triptych protect, detect, treat has been applied here for a long time and has started to bear fruit.
Our territory has an R0 of less than 1, which is one of the lowest contamination rates in France.
- Benoît Payan (@BenoitPayan) September 24, 2020
But this number below 1 is impossible to confirm or deny since the indicators are unveiled at the regional level by the health authorities.
In any event, and this applies to all the indicators, “the values of R should therefore not be interpreted in isolation, but should be put into perspective with the other available epidemiological data and the detailed analysis of the situation. local, ”concludes Public Health France.