Covid-19: red areas, vulnerability, incidence ... how to navigate the maps

An epidemic, and how many cards to analyze its evolution? For several days, the ways of classifying and showing the departments in relation to Covid-19 have multiplied, at the risk of getting lost. A map of the active circulation areas of the virus (ZCA), another built according to the level of vulnerability, to which can be added those based on a single indicator ... Not to mention that they replace those updated during deconfinement, but neglected since!

Here's how to navigate these different maps. They can be classified into four categories, based on ...

... level of virus circulation

Thursday August 27, Jean Castex presented a new list of “zones of active circulation of the virus (ZCA)”, appearing in red. From two departments (Paris and Bouches-du-Rhône) concerned since August 14, we went to 21 at once. Guyana and Mayotte are still declared in a state of health emergency.

To establish which departments go into ZCA, the General Directorate of Health (DGS) grinds and analyzes several indicators of the epidemic (number of tests carried out, test positivity rate, evolution of the number of clusters, hospital pressure, etc.) . However, “the passage in an active traffic zone is not the result of an algorithm, classifying the territory from a certain threshold of the indicators, but of an appreciation of the epidemiological profile of each territory from the different indicators. », Specifies the DGS in Paris.

In other words, if the indicators are "bad" because of a cluster that can be easily mastered, for example, the department will not be classified as ZCA for all that. Conversely, if a department has an incidence rate below the alert threshold, but other elements are worrying or the territory is located next to another in a critical situation, it may be ZCA classified.

This map is intended to be updated according to the evolution of the epidemic, and not at a rate defined in advance. Above all, it has legal weight since the list of departments classified as ZCA is systematically published by decree. The prefect can then take strong measures to fight against the epidemic, such as closing bars and restaurants from 11 p.m. (which was immediately decided in the Bouches-du-Rhône, for example).

... level of vulnerability

Another possibility is to classify the departments according to their “level of vulnerability” (limited, moderate or high). This map, published and updated every evening by Public Health France, is based on "the cross-interpretation of several indicators and reports", indicates SPF in its brochure devoted to "epidemiological surveillance" of Covid-19.

This map partly overlaps with that of the areas of active virus circulation, but not completely. In practice, the main concrete difference is legal. The ZCA "is a category (which is itself based on a contextualized analysis) provided for in law in order to be able to take appropriate measures to combat the epidemic", insists the DGS.

... a single indicator

In addition to these two maps which combine different elements, it is also possible to produce others by taking into account a single indicator among the many existing ones: incidence rate, positivity rate, number of reproduction, number of active clusters, etc. The Géodes site of Public Health France as well as that of the government do moreover daily.

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Take for example the incidence rate, that is to say the number of new cases per 100,000 inhabitants over a week. The vigilance threshold and the alert threshold are set at 10 and 50 respectively. The map shows 21 departments above the alert threshold on August 27. Not all are classified as ZCA (this is the case of the Côte d'Or, for example).

Another possibility: the number of clusters under investigation, these famous “centers of infection”. There are 352 across the country as of August 24, according to the latest epidemiological point from Public Health France. Six departments, which can be arbitrarily shown in red on the map below, have more than ten.

... several indicators (at the start of deconfinement, map now obsolete)

At the start of deconfinement and until June, maps in green, orange and red were also published. Three indicators had been announced initially: the "number of new cases in the population over a period of seven days", "the regional hospital capacities in intensive care", and the "local system of tests and detection of chains of contamination. ".

But this last element was only taken into account with a delay. The map below, stopped on May 7 with a northeast quarter in red, was the first to take all three into account.

On May 28, four indicators were used this time: the incidence rate, the positivity rate, the number of reproductions, as well as the occupancy rate of the intensive care units. Only Île-de-France, Guyana and Mayotte were in orange.

On June 14, the whole map was changed to green, except Mayotte and Guyana. But this map, produced using this method, has not been updated since that date and has been gradually replaced by the others previously mentioned.

This does not mean that the intensive care occupancy rate is no longer taken into account, insist the health authorities. Simply, many indicators related to screening are taken into account more acutely, knowing that many more tests are performed each week in the country (more than 900,000 today). And the interest of the vigilance currently in place is precisely to prevent hospital services, where there is already a slight increase in admissions, from seeing a new influx of many patients.