"This is not a frozen image, the data will evolve and become more precise," warns Simon Cauchemez, head of the mathematical modeling unit for infectious diseases at the Institut Pasteur. The researcher is carrying out, with other scientists, a vast study on immunity in the country.
By focusing on hospital data, combined with international surveys, they mapped the number of people who may have been infected with SARS-CoV-2, as of May 11. This version, the second since the launch of their work in mid-March, was published in the scientific journal Science on Wednesday.
A modeling analysis of immunity to # SARSCOV2 in France this month, as lockdown restrictions ease, estimates low (4.4%) immunity, suggesting efficient control measures that limit virus transmission need to be maintained to avoid a rebound of the epidemic https: // t .co / 6XS5V1isK1 pic.twitter.com/DDogW44msb
- Science Magazine (@ScienceMagazine) May 13, 2020The first report, made at the end of April, predicted that 5.7% of the population, or 3.7 million individuals, would have been infected by May 11. The researchers revised this rate down once their data was updated: to 4.4%, with a margin of uncertainty. "We think that there are between 3 and 7% of the French population who would have been infected," sums up Simon Cauchemez. Or about 2.8 million people on the date of the first day of deconfinement.
Zero R drops 77% with containment
We are far, very far, from the level sufficient to achieve collective immunity which would avoid a second wave without measures to control the epidemic. "It would take about 65% of the population to be immunized for the epidemic to be controlled by immunity alone," recall the study authors.
This small share of infected people is proof that containment has paid off. The zero R (R0), which indicates the number of individuals that a patient can infect, has dropped considerably. It went from 2.9 nationally before containment to 0.67 at the end. Or a reduction of 77%.
"This is the order of magnitude that we hoped to achieve," notes Simon Cauchemez. While there were a lot of uncertainties: the containment was not as drastic as what had been implemented in China, for example. So we were not sure to have such an impact on the transmission. "
Different rates depending on the region
The immunity rate differs by region. According to the researchers' estimates, the areas most affected by the epidemic should also logically register the highest number of infected people and therefore potentially the highest rate of immunization. But these figures remain around 10%. It would stand at 9.9% (margin of 6.6 to 15.7%) for the inhabitants of Ile-de-France and 9.1% (margin 6 to 14.6%) in the Grand Est.
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Next come the Bourgogne-Franche-Comté and Hauts-de-France regions with, respectively, an estimated rate of 4.5% (between 2.9 and 7.6) and 4.3% (2.8 to 7.2 ). The least immunized regions are New Aquitaine with 1.1% (0.7 to 1.9) and Brittany: 1.3% (0.8 to 2.3).
"Once we have the results of serology surveys ( Editor's note: which make it possible to detect the presence of the virus in a group of individuals ) in different places, at different times, we can integrate them into the model and see if mortality is a little higher or a little lower than expected, explains Simon Cauchemez. This will lead us to revisit this map, as this data becomes available. "
"Not enough immunity to lift the measures"
How will the R0 evolve now that the French are deconfining? “As long as it is below 1, the epidemic declines. If the virus were to resume its mad race, we would have to take measures of containment again ”, warned Monday the Minister of Health, Olivier Véran, on BFMTV-RMC.
"There is a lot of uncertainty about what is going to happen now and we are currently trying to set up methods to estimate this number of reproduction in real time in the weeks that follow," says Simon Cauchemez.
One thing is certain: “We think there is not enough immunity to lift the control measures. All the studies say so, WHO too, experts around the world… ”continues the modeler of the Institut Pasteur. “We hope to have treatment quickly to better manage severe forms, and this before 70% of the population is infected with this virus. "
"Avoiding a health catastrophe"
The strategy of letting SARS-CoV-2 spread freely in the hope of achieving collective immunity is, indeed, widely contested. "It's a very, very dangerous calculation," said Mike Ryan, WHO chief of emergency operations, as Sweden appears to be preparing for it.
“This can lead to very brutal arithmetic that does not put people, life and suffering at the center of the equation. Especially since scientists have no certainty that a person is well immunized after contamination.
The alternative chosen by France, which consists in keeping the circulation of the virus as low as possible using control measures, must continue, the researchers believe. "Collective immunity will then take a long time to build," notes Simon Cauchemez. But hospitals will be less overloaded.