“The circulation of the virus relies more than ever on individual responsibility”

To try to “Curb the epidemic” of Covid-19, the executive decided to put in place, since Saturday, March 20, new restrictive measures in sixteen departments (those of Ile-de-France and Hauts-de-France, as well as the Seine -Maritime, Eure and Alpes-Maritimes). The Rhône, Aube and Nièvre could also be affected by restrictions in the coming hours. Can these new restrictions slow the progression of the Covid-19 epidemic?

Mircea Sofonea, lecturer in epidemiology and evolution of infectious diseases at the University of Montpellier, answered your questions in a chat.

Are we in a more serious epidemiological situation than in March 2020?

Nationally no. But, at the regional level, Ile-de-France in particular, the hospital dynamic is approaching dangerously, while some solutions put in place in March 2020 no longer apply (no strict confinement, no mobilization staff from the regions to alleviate the fatigue of healthcare teams).

With an infected population rate estimated at around 35% in Ile-de-France and a first injection rate of 15% in this same region, shouldn’t we soon achieve the start of collective immunity?

Yes, natural immunity as well as vaccination in Ile-de-France will soon make it possible to achieve a level of collective immunity sufficient to constitute a natural brake on the circulation of the virus. It will be a brake, but not yet an obstacle and it will not yet be compatible with a relaxation of all preventive measures, a fortiori because the V1 variant (known as “British”) is more contagious, but also because the virus will still be able to circulate actively in the younger age groups, less vaccinated.

When will we see the effects of vaccination, as it seems to be the case in England?

They are visible when looking at hospitalizations by age group. But we can expect a really significant difference from the month of May – the Pasteur Institute then projects a 50% reduction in hospitalizations compared to a scenario without a vaccine.

However, for each level of immunization coverage there is a corresponding “contact rate” (a degree of social interaction in a way) for which herd immunity is sufficient. The risk is to relax certain measures too early and to return to a contact rate for which the collective immunity acquired by vaccination and natural infections would still be insufficient.

Have new models been carried out taking into account the current restrictive measures, and what are the results?

The two weeks following a change in health policy constitute the blind spot of modeling, a fortiori when the combination of measures is unprecedented (this set of territorialized “braking measures” had not been tested before), for good. and simple reason that we do not have a historical repository with which to configure the model.

It is clear that a November 2020 type containment would have made it possible to end the epidemic in the departments concerned, but this time the circulation of the virus is based more than ever on individual responsibility, and therefore difficult to predict with current methods. Modeling in epidemiology is a very active and progressing field of research, but without data we can only realize more or less optimistic scenarios. The models will, however, be able to start running again at the first signs of the effectiveness of these measures on the case detection data for next week.

The situation in schools in Ile-de-France seems to be deteriorating in recent days. Are schools centers that worsen dissemination?

It is difficult to distinguish the school context from the age group of students, but we know that the youngest can be infected and transmit SARS-CoV-2 and that the closure of schools is a lever for reducing the spread of the virus. If decisions had to be taken – this decision is political, and more broadly democratic – then they would be all the more beneficial if they were applied early, and therefore rather an advancement of the holidays. The question of the extension can be assessed in a second step, depending on the speed of the decline of the epidemic. Distance education is of course desirable within the framework of strict confinement, but the pedagogical and logistical questions are numerous – and, I hope, anticipated.

Once everyone has been vaccinated or immunized naturally, will we still have so many variant appearances?

As long as the virus is circulating somewhere in the world, there will be new mutants. The vast majority of them will not constitute grounds for concern but some may be immunological variants, that is to say capable of bypassing the immune response – a fortiori if the immune memory (of natural origin or vaccine) wears off over time, which is the case with other coronaviruses. When will one of these worrying variants appear? Nobody can predict it because the phenomenon is random.

On the other hand, the less SARS-CoV-2 is brought under control on a global scale, the more quickly this variant can emerge. As for its becoming a “cold”, nothing is less certain: the first SARS-CoV has disappeared, MERS-CoV has existed for almost a decade with more than 30% lethality … But it is a possible scenario, above all if adults are immunized or vaccinated, and the only possible reservoir remains the youngest, with a very low risk of severe form.

Could the arrival of spring and warmer temperatures help curb the circulation of the virus?

Absolutely, sunny days promote life outdoors, better ventilation of homes, all behaviors that reduce the probability of transmission. On the other hand, there is, to date, no biological mechanism demonstrated to link the temperature and the virulence of the virus.

These questions were collected during a chat with the readers of Monde.fr.

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