He climbs, he climbs, until reaching 10.4% this Friday evening.
The positivity rate is one of the indicators of the Covid-19 epidemic being monitored very closely by health authorities and doctors.
In mid-July, it was only 1.2%.
It has now passed the alert threshold, set at 10%, for the first since deconfinement.
While this development is interesting to follow, it should also be carefully analyzed.
We must first explain what this positivity rate is.
This is, over seven days, the number of positive people compared to the number of people tested.
These data are communicated on D + 3, the time that the vast majority of test results are known.
The rate of 10.4% reached on Friday October 9 therefore corresponds to the period from September 30 to October 6.
A negative person twice is only counted once
Warning: contrary to what one might think, it is therefore not the number of positive tests compared to the number of tests carried out.
Which would not be the same.
Because a person tested several times appears only on the first date in the data of the number of people tested.
On the other hand, one always adds a unit in those of the number of tests carried out.
If several samples are reported for the same patient over several weeks or several months, we choose “the first date for the tests with the same result” or that of the “first positive test” if the results are “discordant in the same patient (negative then positive, for example, Editor's note) ”, details Public Health France in its documentation.
Take a fictitious example.
Alain was tested on August 1 and then on October 1.
Each time, the result was negative.
Alain will thus appear on each of these two dates in the total number of tests carried out, but he will only appear on August 1 in the number of people tested.
However, it is this one which is used for the positivity rate.
Alain will therefore not be taken into account to calculate this indicator over any period of seven days including October 1.
READ ALSO>
New daily cases of Covid-19: why so many deviations from day to day?
And cases like Alain, tested several times, are increasing.
"More and more people are being tested, often with a negative result each time", testifies François Blanchecotte, president of the national union of biologists.
We can also observe that the gap between the number of people tested and that of the tests carried out has widened since July.
Prioritization of tests
In addition, the number of people tested per week has decreased since early September.
We went from 1.2 million from September 9 to 16 to 870,000 from September 30 to October 6.
This can be explained by the tensions on the diagnostic capacity of laboratories growing since this summer and by the recommendations on the prioritization of tests, desired by the government at the start of the school year.
VIDEO. Covid-19: 4 hours of waiting for a screening on the weekend, "it's hell"
However, the people now with the lowest priority (those who have to travel or go to see their grandmother without risking transmitting the virus to her, for example) are also, logically, those who are most likely to be negative.
In any case, there are more risks than in symptomatic individuals or contact cases.
Suddenly, if they are less tested, the positivity rate increases mechanically.
"There was a fairly significant communication effort, people who came to be tested just because they were going to see their grandparents, for example, no longer do so", illustrates François Blanchecotte.
An indicator not to be observed in isolation
Under these conditions, the positivity rate should not be taken into account in isolation.
This also applies to each of the other indicators (number of cases, incidence rate, intensive care admissions, etc.).
“The trends should be interpreted with caution,” writes Public Health France in its weekly epidemiological update of October 8.
Newsletter - Most of the news
Every morning, the news seen by Le Parisien
I'm registering
Your email address is collected by Le Parisien to enable you to receive our news and commercial offers.
Learn more
“You always have to take these indicators as a whole.
We discussed this with Jean-François, Delfraissy [the President of the Scientific Council, Editor's note], he totally agrees with us, ”concludes François Blanchecotte.