A new study by the Institut Pasteur reveals that 5.7% of the French population has been contaminated by the coronavirus, far from the 70% hoped to reach collective immunity. On the other hand, thanks to the effects of confinement, the transmission rate has been reduced by 84%, decrypts on Europe 1 Simon Cauchemez, epidemiologist. 

INTERVIEW

A study by the Institut Pasteur, in collaboration with Santé Publique France and Inserm, revealed on Tuesday that 5.7% of the French population has probably been contaminated by the coronavirus. Collective immunity, estimated at 70%, is therefore still far away. On the other hand, the rate of transmission of the virus decreased by 84% thanks to containment. Simon Cauchemez, model epidemiologist at the Pasteur Institute and member of the scientific committee which advises the President of the Republic, reviews this study at the microphone of Europe 1. 

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A falling R0: the effects of confinement 

"We are at 6% at the national level, rather 12% in Ile-de-France or Grand Est, the very affected areas", he specifies. "The lesson is that we are far from the collective immunity that we would need to lift all the measures." The threshold of the latter necessary to avoid a second wave by lifting all the measures is estimated at 70%. "Even if it was 10, 15 or 20%, that would not change the problem. We would be very far from this target of 70%," said Simon Cauchemez. The number of patients without symptoms still raises uncertainties, 20 to 40% of people infected according to different studies. 

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Another important indicator revealed by this study: the R0, the number of basic reproduction, that is to say the number of people that a patient contaminates. According to this study, this R0 is currently 0.5, whereas it was 3.3 before confinement. Currently 10 patients thus contaminate an average of five people, thanks to the effects of confinement. "It is really good news that we have succeeded in reducing the transmission rate by 84%," says Simon Cauchemez. "This explains why we went from 700 admissions to intensive care a day at the end of March to, two weeks later, rather admissions a day. Containment had a huge impact on these admissions to intensive care." 

A drop in admissions to intensive care expected 

With the modeling carried out, the researchers consider that there should be between 10 and 45 admissions per day in intensive care on May 11. "These are serious cases, however we consider that there will be more people infected on the territory at that time, rather between 1,000 and 3,000 cases", specifies Simon Cauchemez. "If we continue to follow the current trend, we hope that the number of admissions will continue to decrease." 

"The difficulty we have in fully understanding this epidemic is that we see the submerged part of the iceberg. Many infected people have mild symptoms that will not be detected by our surveillance systems" , recalls the epidemiologist. The principle of modeling carried out by the Institut Pasteur is used to interpret different indicators to project into the future of the virus.

An effort to maintain after May 11

"If we see 100 dead in France, on the one hand, and on the other hand we have data that describes the probability of dying when we are infected, by combining these two data we will be able to reconstruct history of the epidemic in France, "he explains. To determine the probability of dying from the virus, scientists rely on surveys carried out, for example, on cruise ships.

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A new report will be submitted to the government on deconfinement, scheduled from May 11 gradually. "When we are going out, we must understand that we will not find our normal lives. We will have to maintain strong measures: continue barrier gestures etc but we will have to have an intensive strategy to identify cases , isolate them ... It is a huge effort that we will have to maintain beyond May 11 ".