Jérôme Salomon sees this as further proof of the effectiveness of vaccination.
"The least vaccinated territories have more [positive] cases and those least vaccinated are those where the incidence is the lowest," said the Director General of Health on Sunday, on BFMTV, seeing it as a "very strong argument. ”In favor of vaccination against Covid-19.
What exactly is it?
For our analysis, we used the vaccination data provided by Health Insurance, and the incidence rate (the number of cases per 100,000 inhabitants over the past week) published daily by Public Health France.
These two indicators are based on the place of residence, and not that of the test or that of vaccination, which makes it possible to use the same benchmark.
Less effective against infection
We can see, as Jérôme Salomon says, a correlation between vaccination coverage and the incidence rate. Overall, the higher the first, the less important the second, and vice versa. Even when the epidemic was growing during this fourth wave, in July, this observation emerged. It is surely no coincidence that the incidence reached the highest levels in Polynesia (nearly 3,000), Guadeloupe (2,400) and Martinique (1,200), while the population in these overseas territories is three times less vaccinated than in whole France.
On this animated infographic, we have shown in dark blue Martinique, Bouches-du-Rhône (metropolitan department with the highest incidence rate currently), and Paris.
Overall, we see that the greater the vaccination coverage (therefore the further to the right of the pack), the lower the incidence rate can rise (therefore the lower the point is at a high height).
This correlation is however quite tenuous, probably because vaccines only seem to protect up to 60% against infection, as more and more studies suggest.This is less than the 80 or even 90% that we could expect before the arrival of the Delta variant.
The impact of vaccination on the incidence rate is therefore, inevitably, less important than one could initially hope.
Several limits
But beware: as always, correlation is not causation. And this rather easy analysis comes up against several biases. First of all, it may seem logical that the departments most affected during this 4th wave were those located on the tourist coast, that is to say where the population flows and the opportunities to infect themselves are the most common. more numerous during the summer holidays. However, the departments of the Mediterranean coast, in particular in the Provence-Alpes-Côte-d'Azur region and in Corsica, are among those with the lowest vaccine coverage (less than 70% of inhabitants have received at least one dose) .
In addition, in the event of infection, a vaccinated person will be less likely to have symptoms… and therefore will be less tested.
And conversely: "the implementation of the health pass should lead unvaccinated people (the vast majority not symptomatic) to be tested for non-health reasons", indicates the Department of research, studies, evaluation and statistics (Drees) in a note published on July 15.
Some of them may then be positive even though they did not suspect it.
Which, mechanically, increases the incidence rate.
Read also Covid-19: these pro-vaccination arguments from the government to be handled with caution
Conversely, "there may also be vaccinated people who are no longer careful at all and take more of the risk of becoming contaminated", suggests Rodolphe Thiebaut, professor of public health at the University of Bordeaux (Gironde) and director deputy of the Inserm Bordeaux Population Health research center. "It is very complicated to use this kind of correlation to say if the vaccination is effective, it is better to do clinical trials," he adds. On this point, all studies agree that vaccines protect, especially against the risk of severe form.