Valérie Golaz

INED senior researcher Valérie Golaz tells us about the COVID-19 indicators used in France

What indicators are used to measure the epidemic in France?

A considerable number of different indicators are used to follow how the COVID-19 epidemic is evolving. They range from number of deaths to phone calls made to the “SOS Médecins” emergency health line. RT-PCR molecular tests done in private or hospital laboratories are counted on a daily and weekly basis. Daily indicators, i.e., tests done on the previous 0 to 7 days, are sent from the labs to Santé Publique France, the national public health monitoring agency. Weekly results correspond to tests done over a gliding week. The main statistics are number and proportion of positive PCR tests together with number of new cases and COVID-19 incidence. The number of new cases is the number of people who tested positive over the given period. Incidence is the ratio between that number and 100,000 residents of the area in question. These numbers are regularly diffused by the media, and the public authorities use them to explain and justify their policy decisions. For example, to define the alert levels used in September and October 2020 and explain the decision to apply different policies in different regions of France, incidence figures were cited. New case numbers was one of the indicators used to justify the more recent decisions to lock down France a second time and partially open it again. 

What do molecular tests tell us about the coronavirus epidemic?

Indicators based on those tests clearly illustrate what we know about how this epidemic spreads in the population, and they offer the advantage of bringing together RT-PCR tests done throughout France. However, they are not representative of the population at large. In order for indicators to represent the real magnitude of the virus throughout the population, population categories would have to be tested in the same proportion that each represents in the population at large. The fact is that most of the people who get an RT-PCR test have health risks or symptoms. Moreover, the number of tests done and the number of people tested vary with the policy in effect. For example, a graph from a recent Santé Publique France weekly report shows that the number of tests fell sharply during the two weeks following a speech by the Prime Minister in which he said that contacts indicated by contacts (rather than known infected persons) should not be thought of as contacts (September 21 to October 2; epidemic weeks 39 and 40). Users of follow-up data often forget that at no time do the data come from a representative sample of the population.

How are the data assembled?

Test data are sent electronically through the SI-DEP “screening information” system, a secure platform set up by Santé Publique France to monitor the health crisis. This is an exhaustive dataset that covers all RT-PCR tests done in France. It includes information on person tested—place of residence, sex, age—that makes it possible to map and graph developments with considerable precision. However, as with all data collection, these data cannot be perfect. In some cases information is missing or may have been entered too quickly by the laboratories. Data are either discarded or replaced depending on their type. For example, if the person’s place of residence is missing, it is officially replaced with the laboratory’s postal code. To correctly interpret results, then, researchers need to be aware of all the ways in which the data may be less than accurate.