the indicators to follow to monitor a possible new epidemic wave


The health authorities are worried about a possible epidemic recovery, but it is difficult to navigate the series of indicators that are observed to assess it. Franceinfo takes stock of four of the most relevant data.

Since mid-July, fears of a rebound in the coronavirus epidemic have become clearer. The Minister of Health, Olivier Véran, shared, Monday, July 20, his concern of a “epidemic resumption in some places”. “There is a circulation dynamic of the virus that worries us”, he declared, while specifying that “we are very far from the second wave”. On franceinfo, epidemiologist Catherine Hill estimated that the coronavirus “don’t ask[ait] only to leave “.

>> Coronavirus: find the latest information on the Covid-19 pandemic in our direct

How to monitor the spread of the virus? A myriad of indicators exist: on its site, Public Health France indicates that it is scrutinizing a dozen of them, crossed with other reports such as new sources of contamination. Taken in isolation, an indicator is not enough to gauge the epidemic and sometimes has limits. Franceinfo takes stock and deciphers the interest of the main data taken into account.

The incidence rate, increasing locally

What he measures. The incidence rate corresponds to the number of people infected over a week, reduced to 100,000 inhabitants. Two thresholds have been defined by the Ministry of Health: the vigilance threshold is reached if more than 10 people are infected per 100,000 inhabitants. Beyond 50 infected people per 100,000 inhabitants, the alert threshold has been reached. As the incidence rate is calculated from the results of the PCR tests, it is important to cross-reference it with the positivity rate (proportion of positive tests on all the tests carried out).

What the latest data show. On a metropolitan scale, the incidence rate increases, from 3.9 to 4.3 between week 27 and week 28. As of July 12, two departments had exceeded the vigilance threshold (in orange): the Seine -Saint-Denis (10.06) and Mayotte (17.89). Two others were close to this threshold: Paris (9.87) and the North (9.15). More serious, the alert threshold, him in red, has been exceeded for several weeks in Guyana (321.3), and recently increased in Mayenne, to 52.72. While in this department, the positivity rate was declining.

Why it should be observed with care. The incidence rate has one major limit: it depends on the scope of the population tested. The more tests the health authorities carry out, the more new cases there are, the more the incidence rate increases, without the situation being alarming. However, the screening policy can vary greatly from one region to another and evolves over time. The incidence rate can also change depending on whether the tests more or less target the people most likely to be carriers of the virus.

>> Read also: why we must take the “incidence rate” of the epidemic with a grain of salt

The reproduction rate “R”, on the rise

What he measures. The effective reproduction rate, or R, represents the average number of people that a patient infects. An R of 2.2, for example, means that 10 patients will infect an average of 22 people. When the R drops below 1, the epidemic recedes. Above it grows. Calculated from the number of positive tests in particular, and on the basis of a model developed by the Institut Pasteur, “it makes it possible to analyze the dynamics of circulation of the virus, in an epidemic context”, note Public Health France on its site. “In a period of low circulation of the virus, the R is more of an alert indicator to identify an unusual situation”, continues the agency.

What the latest data show. According to the figures for the week of July 13, the reproduction factor exceeded 1.5 in two regions: in Provence-Alpes-Côte d’Azur (1.55), but especially in Brittany, where it reached 2, 62, down from 1.07 the week before. It is also 2.26 in Reunion. It remains below 1 in three regions and varies between 1 and 1.5 in six others.

Why it should be observed with care. The limits of the reproduction factor are similar to those of the incidence rate. On its website, Public Health France explains that “the value of R can increase without this necessarily being a sign of an intensification of the circulation of the virus”. For example, the discovery of a cluster in a company can lead to a certain number of positive screenings, occasionally increasing the R without there being a wider spread of the virus. This is the case in Brittany, where the R increased following the discovery of several outbreaks. In addition, the very volatile reproduction factor is the result of a complex calculation which attempts to model, and therefore simplify, human behavior. It therefore does not accurately reflect reality and must be contextualized with other indicators.

>> Read also: Why the reproduction rate of the virus is an indicator to be taken with caution

The interventions of SOS Médecins, in slight rebound

What they measure. Since the beginning of March, the network of SOS Médecins associations has recorded the number of acts for suspicion of Covid-19 carried out each day by their doctors. This data makes it possible to have a daily look at medical acts outside hospitals and practices.

What the latest data show. In the week of July 6 to 12, this number was up for the third week in a row, recalls Public Health France. This increase concerned all age groups and all regions of mainland France. We observed a peak on July 14, with 576 interventions, before a slight decrease the following days.

Why this should be observed with caution. The interventions of SOS Médecins for suspected Covid-19 only make it possible to monitor part of the reaction to a possible epidemic rebound. They are to be completed with what is observed every day at the entrance to hospitals.

New hospitalizations and resuscitations, which barely shudder

What they measure. If there was a new wave, it would finally be seen at the doors of hospitals. The new hospitalizations for Covid-19, as well as the new admissions in intensive care, highly scrutinized at the height of the epidemic, are very telling indicators of a possible resurgence of the epidemic.

What the latest data show. At the national level, the number of admissions to hospital is not increasing. It was 606 in week 28, up from 646 the week before. But admissions in intensive care are up very slightly: there were 78 new patients in intensive care, against 73 the previous week. At the regional level, except Guyana or Mayotte where the epidemic is still intense, these figures barely shudder, for the moment.

Why this should be observed with caution. This stability of recent data is not sufficient to prove the absence of a new wave. If this were to occur, it would probably manifest itself first in test-related indicators and then, ultimately, in hospital data.

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