On March 12, when Emmanuel Macron decided to confine the country, he read the long-awaited opinion of his scientific council the same day. In addition to analyzes by French researchers, the note written by the British star “modeler” Neil Ferguson, from Imperial College London, caused a sensation. The learned calculations of the prodigal mathematician estimate that "several hundreds of thousands of dead" are expected in France if nothing is done to curb the virus.
In England, the researcher, who will eventually fall ill with Covid-19, has set the bar at 500,000 deaths. It is his forecasts that will also force Boris Johnson, the British Prime Minister (himself ill) to take additional precautionary measures to try to stop the spread of the virus.
If no one today seriously contests the effectiveness of containment to roll back the Sars-Cov 2, it is now proven that these projections were too pessimistic. So much the better! But that throws a harsh light on the strengths - and weaknesses - of epidemiology, the discipline that is supposed to predict the spread of disease.
Things in common with the weather
Every evening for weeks, the French religiously listened to the DGS, Professor Jérôme Salomon, shouting out the figures for the day's hospitalization as a weather presenter presented the temperatures. And its map in "green" and "orange" of the departments had false airs of that of Météo France. The parallel does not stop there. "It is true that epidemiology resembles meteorology. It fascinates everyone to know what the weather will be like… and whether an epidemic will progress or not, because in both cases we are all concerned ”, philosopher the eminent epidemiologist William Dab.
The science of the "frog" skinned complex scientific data (displacement of the air masses responsible for rain and good weather ...) to make bulletins ... more or less reliable. Likewise, epidemiology must take into account multiple factors - the rate of contagion of the virus, the number of people potentially affected, the speed of spread - which are sometimes simple hypotheses, in order to arrive at a prognosis for evolution or to predict future mortality. A "crystal ball" which can be perilous, because the unknowns and the working hypotheses are numerous.
“In France, we are very Cartesian and we want affirmed results. Even for the weather, will it be nice or not? In the United States, we are more probabilistic. Newsletters say it’s likely to rain 60%. We should take the same caution when looking at models for predicting the progression of the disease, and not take them for certainties, ”argued Professor Dab.
"On mortality, we were all wrong"
Epidemiologists, good scientists, are starting to learn from this experience. "On mortality, we were all wrong, me first," admits frankly Catherine Hill, epidemiologist.
Reason seems to make sense today, but it was much more complicated to imagine in March. “At the start, we had a problem calculating the case fatality rate. We knew the number of deaths, but we did not know how many people were infected. So we took into account the number of cases reported. This initially gave very high case fatality rates, much higher than reality. "
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It was at this time that Ferguson's models imagined "several hundred thousand deaths in France". The first forecasts of the Institut Pasteur mathematical models are on the same line. To arrive at these gloomy forecasts, they assume a high case fatality rate and a large circulation of the virus. However currently, and subject to a second wave, there are in France "only" a little more than 29,000 dead.
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How to explain such a difference? The novelty that completely changed the situation is the presence of a very large number of asymptomatic people, which was not at all envisaged at the start. “It changed all the calculations. There were still as many dead, but reported to a population suddenly affected much larger, the fatality rate was crumbling, "said Catherine Hill, who looked closely at dozens of studies on the subject.
“Our science makes us modest. I prefer epidemiology close to the daisies, by reading the publications saying what really happens in hospitals, rather than the mathematical projections which depend on so many hypotheses that they are likely to turn out to be false. "
"The rate of contagion, we saw it"
However useless to throw the epidemiologists at the nettles and it is besides for that also also that their matter interests so much. They did identify one aspect of Covid-19 that turned out to be early: its high rate of contagion. “I said on a TV set in early March that the number of cases would double every three days. We had 1000 cases on a Monday, that meant 2000 cases on Wednesday, 4000 cases on Friday ... There people looked at me puzzled, almost skeptical ... but it was true, "claims Professor Dab.
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“The main mistake with Ferguson-type models was that the Covid-19 would spread evenly, like the flu does. But this initial assumption was false. We now know that the Covid-19 spreads locally, from super clusters, like that of the Evangelists in the Grand-Est or the Creil base in the Oise, and that to combat them, contact cases must be identified and isolate them, which the public authorities are currently doing. This is real effective work as an epidemiologist in the field, ”he adds.
“Epidemiologists are especially strong at dealing with crises once they are over. They never agree with each other… ”tackle, stern, a senior official in the world of health.
The stars of the health forecast
Neil Ferguson: the disavowed English prodigy. A few weeks ago, her star was in the sky, but she grew pale. The star mathematician of Imperial College London whispered in the ear of the powerful his gloomy predictions of hundreds of thousands of deaths to come. But his calculations have, fortunately, turned out to be wrong, out of ignorance of the numerous cases of asymptomatics. The gossip reminds him that he had already predicted, wrongly, that hundreds of thousands of Britons would die from mad cow disease. If he himself fell ill with Covid-19, it was fortunately only mild.
William Dab: the return of the old sage. Former Director General of Health, following the heat wave crisis in France in 2003, this affable epidemiologist knows perfectly the workings of an administration which has not been exempt from all reproaches in the face of previous health crises. While he had placed himself a little behind, his expertise reappeared on TV sets in terms of the Covid-19. Pr Dab was not however favorable, in March, to the maintenance of the municipal elections, and criticized the shortage of tests and masks. He participated in a ministerial mission on the spread of the epidemic in Mayotte.
Catherine Hill: From the pick to the study of the Covid-19. She usually follows the epidemiology of cancers for the Gustave-Roussy Institute. Previously Catherine Hill made herself known by carrying out a large investigation in 2010 on the deaths of the Mediator drug. During this Covid-19 crisis, she carefully analyzed the dynamics of the pathology, closely studying the numerous international studies that were coming out on this subject. With her freedom of tone, she does not mince her words in the face of certain mistakes by her colleagues.
Antoine Flahault: the return of the doctor Sentinelles. This teacher's advice is widely listened to. This great connoisseur of the dynamics of epidemics has a great deal of experience in the field, for he led the team of doctors "Sentinels" from Inserm for a long time, responsible for monitoring the evolution of the seasonal flu epidemic. He is a doctor of medicine and biomathematics, and currently teaching at the University of Geneva.