• Since this Friday, the tests are no longer free in France for part of the population, leading to a drop in their use.

  • As a result, the number of new cases in France is stagnating, while the epidemic was on the rise.

  • Does the drop in tests hide the evolution of the epidemic from us?

Since this Friday, October 15, the so-called comfort tests are paying in France, after having been free since the very beginning of the coronavirus epidemic. Tests are still reimbursed for the majority of the population, namely those vaccinated (74% of the total population), children under 12), as well as in a number of exceptions (medical prescription, contact case, positive test recent, contraindication to the vaccine).

It is still nearly seven million unvaccinated adults (or with a single dose for the moment) who will potentially have to pay to have their noses tampered with.

And as one could imagine, this is not without consequence on the number of tests carried out.

On Friday 15, the number of tests fell by 45% compared to the previous Friday, from 675,075 tests on Friday 8 to 371,174.

Rebelote on Saturday, with only 289,638 tests against 525,646 on Saturday 9.

Fewer tests, fewer cases, less visibility

The Ministry of Health expects two million tests to be performed each week despite the end of free, while nearly 3.2 million tests were performed the week of October 10 to 14. Will this drastic drop in the number of tests prevent correct monitoring of the evolution of the epidemic? The week-by-week evolution of the average daily number of cases is one of the first criteria to assess the situation in France, because hospitalizations arrive about two weeks later, which makes the indicator, although more precise, very long. future. The first wave is a good example: at a time when only patients were tested, the time to see the saturation of hospital beds, it was far too late.

However, the fewer tests there are, the fewer cases there are, which is simply the logic of things.

And there is no lack of it: while the number of cases has been on the rise for several times, raising fears of the start of a new wave, cases have stagnated since… Friday and the reduction in tests.

"The comparison with the recent period becomes impossible, it will probably be necessary not to judge the cases on the same frames of reference as before to see how things evolve", estimates the doctor Franck Clarot.

Unreliable tests in the background

All the more so since the All Saints' Day holidays arrive in a few days, which will again create a disruption in the number of tests: generally a lot the day before departure, followed by a drop during. "There will be a double change between the end of the free education and the holidays, which risks making the epidemic all the more illegible" warns the doctor. The number of tests varied enormously during the epidemic: almost non-existent during the first wave, massive during the second, lower in May with the number of public holidays, before exploding completely during the summer afterwards. the health pass.

Relying less on tests is not really a loss according to Samuel Alizon, evolutionary biologist at the CNRS in Montpellier, for whom "the variable of positive tests in France has always been extremely biased".

Depending on the number of tests carried out, but also on their use (people do not test themselves in the same way before and after the health pass), the target population, not to mention the unreliability of the antigens, the reasons for calling into question. cause this crude number of cases abound.

The certainty of the random

With fewer tests carried out, the researcher pleads for another approach.

While currently, we mainly test people who have symptoms, he advises instead "random sampling."

The British have been doing it for more than a year and thanks to it they have a much finer picture of the epidemiological dynamics.

But this requires the involvement of people with good expertise in statistics in order to design the sampling plans ”.

In the meantime, "we must be more vigilant to weak signals insofar as we test less.

The slightest rise must be analyzed, ”insists Franck Clarot.

A good reason to be positive anyway?

With the end of free tests, vaccination should increase a little in France, underlines the doctor.

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