The return to school is looming, and for the first time in a long time, Public Health France (SpF) uses the term “Exponential” to qualify the spread of Sars-CoV-2, even suggesting a transmission dynamic “Very worrying” in its latest weekly newsletter. “The more time passes, the more things become clearer and the more we can ensure that we are not going towards a plateau”, specifies Daniel Lévy-Bruhl, epidemiologist at SpF. For the week of August 17 to 23, all indicators are on the rise: increase in the number of confirmed cases (+ 57% compared to the previous one), the positive rate of samples (+ 3.7%), or even the number of clusters (192 in addition).
For the first time this summer, the increase concerns all regions. In 14 metropolitan departments, the incidence (the number of positive cases per 100,000 inhabitants) even exceeds the alert threshold of 50, especially in Île-de-France and the Paca region, the highest rate being observed in Bouches-du-Rhône (140 / 100,000). SpF notes that the number of people tested is increasing but not at the rate of progression of confirmed cases. This increased screening alone does not explain the increase in the number of cases.
From the moment a patient infects more than one person on average (the reproduction rate is 1.38 for the week of August 17 to 23), the exponential nature of the epidemic is a reality. And this has been the case for several weeks already. For Mircea Sofonea, lecturer in epidemiology of infectious diseases at the University of Montpellier, there has been no rupture in recent days: evoking the character “exponential” of the epidemic is “Just a way of saying” beware: we are for the moment in the slow phase of the exponential curve, after a while, we will be in the explosive phase “”.
When? Difficult to predict, obviously, but we could expect, according to him, to fill 5,000 resuscitation beds (which was the pre-epidemic capacity) at the end of October, beginning of November. As he recalls, “Exponential nature is not synonymous with speed”, and the increase in the number of cases is largely slowed down by health measures that allow “Shift the deadline by a peak and the saturation of hospitals”. For now, the number of cases detected is doubling every 14 days, well below what we knew before containment.
However, beware of“Back-to-school effect” which brings together several conditions favorable to an acceleration of the epidemic. Resumption of socio-professional, academic, sporting and cultural activities, back-to-school purchases, administrative procedures … “So much ‘fuel’ for the virus” according to Mircea Sofonea, not to mention the arrival of cold and days spent in a poorly ventilated enclosed environment. “No one can quantify this ‘re-entry effect’, but it is obvious that it is not going to improve”, he suggests. Especially in large cities.
In its weekly report, SpF notes that “More than half of the positive cases (51%) presented symptoms” during the week of August 17 to 23, compared to 48% the previous week. “An indicator that is difficult to interpret but which could be partly good news, if it corresponds to the fact that subjects presenting symptoms will resort to tests more”, suggests Daniel Lévy-Bruhl.
As throughout the summer, the weekly increase in the number of cases remains in any case greater among young people (+ 67% among 15-44 year-olds), but it now concerns all age groups, with + 43% of cases among 65-74 years old and + 13% for those over 75 years old, considered as populations at risk. As the SpF epidemiologist explains, “During the summer, a lot of contamination occurred between young, less vulnerable subjects, which largely explains why this increase in cases did not translate into such a clearly negative dynamic in terms of hospitalizations”. But the family reunions of this period are factors of intergenerational transmission, recalls Mircea Sofonea, and “If the virus circulates a lot in an age group, mechanically you will then see more cases appear in those in which the virus has circulated less”.
“The main criterion of concern is not so much the cases as the consequences in terms of hospitalizations”, recalls Daniel Lévy-Bruhl, and “There is currently no argument to think that the virus is different” of what he was. SpF has already noted an increase in new hospitalizations (+ 7.5%) and intensive care admissions (+ 36%, to 174), even if it remains at limited levels.
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