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These four indicators to know before fearing a second wave of epidemics


INDICATORS – Mayenne, Gironde, Finistère, Ile-de-France … epidemic indicators are rising again but what are we talking about? What does an “alert threshold” or “incidence rate” mean? LCI helps you understand better.

Is France threatened by a resumption of the coronavirus epidemic? Several departments in the west are under surveillance. Mayenne in particular has been so since it exceeded its “warning threshold” for the presence of the virus, massive test campaigns have been launched there and wearing a mask is mandatory in public places. closed. In July in Brittany, the “reproduction rate” of the virus jumped … without this indicating a sharp increase in cases. Warning threshold, alertness threshold, incidence, reproduction rate … what do these terms mean to qualify the epidemic? And from what “threshold” can we consider the epidemic starting again? Here are some answers to better understand and not mix everything.

accident frequency

This corresponds to the number of positive cases compared to the population. This incidence reaches its alert limit when there are more than 10 new positive cases per 100,000 inhabitants in the last seven days. The threshold then changes to “alarm” when there are at least 50 new cases per 100,000 inhabitants for seven days. It is for this reason that with 143 new cases per 100,000 inhabitants on July 30, Mayenne is still well above its “warning threshold”. On that date, 17 other departments had an incidence of between 10 and 20 new cases in the last seven days, with Val d’Oise presenting 20.5.


This indicator applies this time to the so-called PCR tests, ie those that are performed to diagnose whether a patient is positive for the virus or not. It is these tests that are performed with swabs inserted into the patient’s noses. The degree of positivity for PCR tests exceeds its alertness threshold when it is between 5 and 10% and its warning threshold above 10%. Nationally, the positivity was in mid-July, 1.4% according to figures from Public Health France on 30 July.

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Reproduction rate or “effective R”

This frequency, also called “effective R”, corresponds to the number of people around him that a patient pollutes. It is the result of three factors: the risk of contamination during physical contact, the number of daily contacts and the duration of a sick person (up to 14 days). This “R” reaches its alertness level between 1 and 1.5 and its warning level above 1.5. Specifically, the epidemic returns when it is less than 1 and it accelerates when it is greater than 1. To calculate the effective R, Santé Publique France is based on the screening tests performed in each region.

The map below, produced by our partner Esri, shows the “effective R” region by region. In the Center Val de Loire, it is currently 2.27.

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Hospital Capacity

The last indicator, hospital capacity, is defined by the number of resuscitation beds available. This level exceeds its vigilance threshold when intensive care beds are occupied by between 40 and 60% of Covid-19 patients and its warning threshold of more than 60%.

Indicators that must be combined

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Reproductivity, prevalence, positivity … none of these “lone” indicators allow us to conclude that the epidemic has resumed. The “effective R”, for example, is useful for monitoring viral circulation. But it is not enough to see the epidemic accelerate, explains Santé Publique France. In fact, the reproduction rate can increase rapidly due to several events specific to each territory: “For example, the appearance of a cluster in a company can lead to screening measures and an influx of patients in an emergency department or in a laboratory, and sometimes increase R without there being a spread in society.“.

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