Covid-19: why we should expect a rebound in contamination

A halt to the drop in the number of cases. Since the peak of January 24, new daily contaminations have fallen very quickly in France, falling in one month from 365,366 to 62,146*. But in recent days that trend has slowed, until it appears to be plateauing, at a level just slightly above 50,000 infections per day. Should we see in this the premises of a new rebound? “In several European countries, the epidemic is no longer decreasing. We are even sometimes witnessing a fairly rapid rise in contamination. This is the case in particular in Finland, the Netherlands, Switzerland, as well as, to a lesser extent measurement, in the United Kingdom, Italy, Germany and Austria”, notes Professor Antoine Flahault, epidemiologist and director of the Institute of global health at the University of Geneva, in Switzerland.

In France, we are not quite there yet: “A few weeks ago, ten infected people contaminated seven, and the epidemic was regressing. Now they contaminate rather nine, or even a little more: the number of cases may already be growing slightly, because screenings always arrive out of step with infections”, indicates Mircea Sofonea, epidemiologist and lecturer at the University of Montpellier. Under these conditions, Professor Flahault assures that he “would not be surprised” if France experienced in the coming days a rebound in contamination similar to that recorded among our neighbors, quite logically followed a little later by a new increase in hospitalizations.

Decline in immunity

This trend reversal can be explained by several closely intertwined factors – all of which are set to persist in the weeks to come. First, the rise of the Omicron BA.2 sub-variant. The example of Denmark had however given hope that this newcomer would not relaunch the circulation of the virus: “In this country, BA.2 arrived very quickly after BA.1, but it simply delayed the drop in the number of The peak was expected on January 15, and it arrived at the end of January. On the other hand, more than a month after the peak of contaminations, mortality remains very high in this country, it is even three times higher than that recorded in France at the moment”, recalls Professor Flahault.

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This variant, more transmissible than BA.1, is now beginning to impose itself in other countries, and in particular in France where it already represents 38% of contaminations. What to relaunch the epidemic at home? Everything will depend on the duration of the protection conferred by BA.1 vis-à-vis its successor. “The British tell us that reinfections with BA.2 after BA.1 are marginal, but they remain possible. And above all, this immunity will decrease over time, especially for people who have had mild illnesses”, notes Mircea Sofonea .

In addition to the heavy circulation of BA.2, there is the end of school holidays and the reopening of schools. “What we see in France is very remarkable geographically: the regions where there is already a real rebound, such as Brittany or Normandy, correspond to the areas where prices have resumed for a fortnight”, notes the Professor Flahault. The under 11s being, in France, much less vaccinated than in other countries, it is not very surprising that the resumption of contamination is done in the first place through schools. “If there were to be a significant wave again in children, this would then reverberate in adults. It must be borne in mind that there are always children and adults at risk of severe form, due to comorbidities or immunosuppression”, underlines Professor Flahault.

General relaxation

But it is above all the general relaxation of barrier measures, in France as in other countries, which explains the current evolution of the epidemic. “It was expected, given the optimism distilled by the executive, by certain experts and by the media, with the promotion of the lifting of almost all the control measures which will be ratified in mid-March”, notes Mircea Sofonea. And this is only the beginning: with the war in Ukraine and the presidential election, we collectively have other concerns, with the wish to put behind us these two very trying years. “The same phenomenon was observed with the Spanish flu: after the third wave in 1919, the epidemic was considered over, while there were still substantial peaks afterwards, with many deaths”, notes Professor Renaud Piarroux, epidemiologist and head of the parasitology department at the Pitié-Salpêtrière hospital.

A state of mind that also plays in favor of the virus. Because the French, wanting to move on, now seem to be less tested: “This can be seen very clearly in the positivity rate, which remains at a high level and hardly drops anymore”, specifies Renaud Piarroux. Viral circulation is therefore probably underestimated, with some of the infected people who no longer screen themselves and do not isolate themselves either…

“In the short term, however, there is no reason to have major concerns. Even if there is a rebound, it should remain of a limited magnitude”, indicates Mircea Sofonea. But beyond a few weeks, it is the unknown: if the slackening proves to be massive, the summer brake, real but modest, will not be enough to prevent the continuation of the increase in the number of cases. Especially since the decline in immunity should continue: “Nothing is being done to maintain it, since the option of the fourth dose has not been retained for the moment, when we know that the protection conferred by the vaccine against infections is not very durable”, underlines Professor Piarroux.

Most experts therefore plead for an evolution in the management of the epidemic. “In addition to the current tests, which should be continued, random screenings should be carried out in the population, to have a more precise idea of what is circulating”, suggests Mircea Sofonea. Without forgetting the implementation of virus control actions that are less painful than masks and social distancing: measurement of carbon dioxide in closed places and ventilation. A necessity, to avoid too brutal a backlash on the Covid front.