Covid-19: in France, only one in twelve positive cases would be detected

All the specialists say it: to hope to control an epidemic, it must first be measured. But in this game, France is doing much worse than some of its neighbors. It’s not a new problem. “Since the start of the Covid-19 pandemic, we have only detected around one in three positive cases”, epidemiologist Catherine Hill recently explained to L’Express. And over the recent period, it is worse: only one infected person in twelve would be identified, if we are based on the recent results of epidemiological studies carried out in the United Kingdom. In other words, instead of having recorded nearly 90,000 new cases per day (official figures for the period from March 14 to 20), France should, in reality, have recorded more than a million daily cases. in his records!

This problematic discrepancy between our statistics and the real situation is due to our registration system, which is entirely based on the number of tests carried out. Like a pierced net, it lets many cases escape. “The first limit is logistical: we cannot test everyone. On March 24, we had a daily screening rate of 0.63% in France. This means that we do not even test 1% of the population daily. “, notes Claude-Alexandre Gustave, medical biologist in the Lyon region.

At the same time, asymptomatic people are less likely to appear in the registers, since the French generally wait to have symptoms before checking if they have been infected. “Finally, over the recent period, the population has increasingly resorted to self-tests or antigen tests rather than PCR tests. However, we know that these methods lack effectiveness. Antigens, for example, have a sensitivity about 50% to 60%. This means that by using them, one out of two infected people is not detected. For the self-tests, it is worse, because the sampling is not always carried out correctly. moreover, their results do not appear in the official data, because very often, the users do not take the step”, explains Claude-Alexandre Gustave.

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To get a clearer picture of the situation, epidemiologists have their own tricks. They can, for example, divide the number of cumulative deaths by the number of cases found and compare the results from one country to another. “Last January, we had a number of deaths to number of cases ratio of 0.3% in Denmark and 0.8% in France. If medicine is as effective in both territories, we can deduce that the number of cases is underestimated in France by a factor of 3″, explains Catherine Hill.

A gap of 1 to 20 in Wales

But over the recent period, the gap between the official statistics and the real situation has increased further, thinks Claude-Alexandre Gustave, who has just combed through epidemiological studies conducted across the Channel. “In Northern Ireland, England, Wales and Scotland, the National Statistics Office sends test kits every fortnight to a fraction of the population deemed representative,” explains the specialist. These results are then aggregated across the country. This makes it possible to have a reliable estimate of what is happening in the field, because the study includes all age groups. It also mixes professions, places of residence, immune characteristics… The conclusion of this work? The number of cases recorded is largely underestimated: 12 times in England and 20 times in Wales over the recent period.

“What we are seeing for England can no doubt apply to France. So far, the two countries are facing a similar epidemic. Admittedly, there is a slight time lag, but the variants, the rates of mortality and vaccination are roughly the same,” said Claude-Alexandre Gustave.

How can this growing underestimation be explained? First by the characteristics of Omicron. “Since its arrival, a real change has taken place. Its transmissibility and its immune escape contribute to the circulation of the virus. This mechanically increases the underestimation factor. However, we must not forget the behavior of the populations. Today , many people say to themselves that catching Omicron is not serious. There is therefore no longer any need to worry about the Covid or to be tested. Finally, the use of self-tests in a private way also distorts the view given by published data.

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However, this situation carries risks. “The virus is not going to disappear. We will have to live with it, but between pretending it is not there and keeping adaptation measures in place such as ventilation or wearing a mask in an enclosed place, there is a world. The consequences, from a health point of view, are not at all the same”, recalls the expert. Certainly, in the immediate future, the situation seems under control: there is no need to reconfine, the hospital does not collapse… But by letting the virus circulate, we accelerate its evolution. And therefore, the risks that it will be against us.”