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declared cases, positivity rate, closed classes … How to navigate among the figures on the epidemic in schools?

Should all schools be closed for fear of an outbreak of Covid-19 cases? Are schools places where the virus is transmitted? Are children less infected than adults? Faced with these recurring questions, many figures are summoned, sometimes difficult to interpret.

>> Covid-19: follow the latest information in our direct

While the latest figures communicated by the Ministry of National Education on Friday, March 26, show a clear increase in the number of infected students over a week, franceinfo takes stock of the various data available.

Positive cases in schools

Where does this figure come from? It is provided by the Ministry of National Education and is based either on statements from students or staff, or on tests carried out in schools. Over the last seven days, 21,183 students have declared that they had tested positive for Covid-19, or 0.17% of the total students, according to a press release from rue de Grenelle dated March 26. Among the personnel of the National Education, 2,515 declared that they had tested positive during the week, that is to say 0.22%.

How to interpret it? The rates provided by the National Education can in no way take into account the real number of cases of Covid-19 in schools, since they only take into account on the one hand the declared cases and on the other hand those identified. thanks to tests in schools, far from being systematic. Between March 15 and 22, 200,404 tests were carried out while the National Education has more than 13.5 million students and staff. Among these tests, 0.49% were found to be positive, according to the ministry.

The number of closed classes and establishments

Where does this figure come from? It is provided weekly by the Ministry of National Education. And it is on the rise, according to official figures, with some 148 schools currently closed against 80 a week ago and 30 two weeks ago. Finally, dn the establishments that remain open, 3,256 classes are closed out of the 528,400 classes in France, ie 0.6%. This number is also up from the 2,018 classes closed a week ago.

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How to interpret it? Although scientifically very imperfect, this indicator provides an idea of ​​the upward or downward trend in the epidemic in schools. From next week, the comparison will be less relevant because of the strengthening of the health protocol announced by Jean-Michel Blanquer: indeed, from March 29, in the 19 reconfigured departments, a class will be closed from the first case of contamination when three were previously needed.

The incidence rate

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Where does this figure come from? In general, and regardless of age, the incidence rate is the number of people who tested positive over a rolling week per 100,000 inhabitants. On Thursday March 25, this incidence rate was 325 for France, according to the CovidTracker site, which is based on official data from Public Health France, an agency under the Ministry of Health. That is to say a very high rate, knowing that the alert threshold is 50.

There is no figure for the incidence rate in schools. However, Public Health France is raising the incidence rate by age group. In the week of March 15 to 21, 2021, this rate was 431 among 15-17 year olds, corresponding to high school level, according to the latest epidemiological bulletin of the health agency. The incidence was measured at 350 for 11-14 year olds (middle school), 283 for 6-10 year olds, 122 for 3-5 year olds, and 58 at the 0-2 years old.

On average, contaminations of moins de 18-year-olds increased by 28% from the previous week. The infographic below, which traces at the national level the evolution of the incidence rate among young people since May 2020, shows a continuous increase in recent weeks.

Curve of the incidence rate (number of people tested positive per 100,000 inhabitants over one week) by age group since May 2020 and until the week of March 15 to 21, according to data from the Public Health France agency in its epidemiological bulletin published Thursday, March 25. & nbsp;  (PUBLIC HEALTH FRANCE)

How to interpret it? Does this sharp increase mean the epidemic is out of control in school, college and high school? ATAttention, specifies Public Health France: “This figure fits in a context of a significant increase in the screening rate, partly linked to the intensification of screening campaigns organized in schools. “ It is therefore possible that we now screen more for the virus in schoolchildren, college or high school students who were not tested before, due to lack of symptoms.

The positivity rate

Where does this figure come from? The positivity rate corresponds to the number of people tested positive (RT-PCR and antigen test) compared to the total number of people tested positive or negative in the same period and if this rate is greater than 10%, it is classified as “red”. As for the incidence rate, this indicator is calculated by age group, and not strictly speaking in schools. In the week of March 15 to 21, this indicator, ranging approximately between 4% and 9% for those under 18, depending on the age group, is rather stable or declining, says Public Health France. More precisely, he was stable at the 0-2 years and decreasing in the orres age groups (between -2% for 15-17 year olds and -14% for 3-5 year olds)

The change in the positivity rate (number of positive tests out of the number of tests carried out in total) since May 2020 by age group in the youngest, according to the epidemiological bulletin of Public Health France as of March 25, 2020 (SANTE PUBLIQUE FRANCE) )

How to interpret it? The study of the positivity rate makes it possible to put into perspective the evolution of the incidence upward or downward by taking into account the number of people screened. Last week, more tests were done, but a lower proportion of them tested positive.

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