why the return to normal life now requires 90% of vaccinated adults

Published on : 07/04/2021 – 18:41

A new study, published Tuesday on the Institut Pasteur website, concludes that due to the so-called “English” variant, 90% of the adult population should be vaccinated before they can return to a normal life without risk of rebound epidemic.

Fichu variant B.1.1.7, called “English”. Its spread on French soil makes hope very precarious, mentioned by some scientists at the start of the year, a return to “life before” the pandemic in the fall of 2021, according to the results of the new models published on the Institut Pasteur website, Tuesday April 6.

With an English variant that has become dominant in France, 90% of the French adult population would need to be vaccinated against Covid-19 at the end of the summer to be able to wipe out all social distancing measures without causing a new outbreak. the epidemic, conclude the authors of this study.

The “English” variant made people jump the reproduction rate of the virus

What – between the problems of delivery of doses, the official vaccination schedule of the government, and the reluctance of part of the population – is an almost impossible objective to achieve according to several specialists interviewed by France 24. “The The main message of our work is that vaccination will certainly enable us to emerge from the crisis, but that we must still expect to live with certain constraints this fall ”, underlines Pascal Crépey, researcher in epidemiology and biostatistics at the School. graduate in public health in Rennes and co-author of the study published on the Institut Pasteur website, contacted by France 24.

So how did these researchers achieve this 90%, when it was still recently admitted that it was necessary to aim for a protection rate of the population against the virus of between 60 and 70% (the famous collective immunity) to put wrong with the Covid-19 epidemic?

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To understand it, you have to realize how much the B.1.1.7 variant has changed the situation … and more specifically its impact on the famous R0, that is to say the average number of people infected by a case. in the absence of immunity and without measures to control the epidemic. “The R0 of the majority strain circulating in France has changed. It was around 3 for the historical virus and it is estimated between 4 and 5 for the ‘English’ variant ”, explains Amaury Lambert, professor of mathematics at the University of the Sorbonne, contacted by France 24. This jump in R0 was calculated by “taking into account scientific estimates according to which the ‘British’ variant spreads 60% more easily”, explains Pascal Crépey.

If children could be vaccinated

From there “this automatically increases the vaccination threshold to be reached in order to be able to envisage a return to normal life without the risk of an epidemic rebound”, concludes Amaury Lambert. This is how we went, with the study model put forward by the Institut Pasteur, to an objective of 90% of people over 18 years of age vaccinated in the central scenario of the study which makes of the “English” variant, the majority strain in France.

The researchers recognize, however, that this threshold to be reached would be lower if the children were also vaccinated. Indeed, “if only adults are vaccinated, a significant epidemic is still expected in children, contributing to the infection of parents and unprotected grandparents”, note the authors of the study.

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By including those under 18, “the vaccination of 60-69% of 0-64 year olds and 90% of over 65s could allow the complete relaxation of control measures” from September, acknowledge the authors of the study. .

But this is a purely theoretical discussion since at present “no vaccine has a marketing authorization for children in France”, recalls Pascal Crépey. Indeed, the results of clinical tests to prove that the remedies of Moderna, Pfizer or AstraZeneca are as effective and safe in minors as in adults have not yet been published. “It is obvious that if we knew the effectiveness of these vaccines in children, this could make it possible to adapt the vaccine strategy”, summarizes Jean-Stéphane Dhersin, deputy scientific director of the National Institute of Mathematical Sciences and specialist in epidemic modeling, contacted by France 24.

More protective vaccines?

Another parameter that could influence the priorities of the vaccination campaign is the level of protection provided by the injections. “Initially, we knew that vaccines greatly reduce the number of severe cases of Covid-19, but there was a lack of data to know whether they were as effective in limiting transmission [l’infectivité], or to prevent contamination [la susceptibilité] of the vaccinated person. This is why it was decided to vaccinate the frail and elderly as a priority in order to limit the number of deaths and hospitalizations ”, recalls Pascal Crépey.

Observations since the start of vaccination campaigns around the world suggest that the vaccines available also offer good protection against the risk of contamination. The model adopted by the study indicates that, in the case of a vaccine effective in preventing severe forms and which prevents infection, “the order of priority of the populations to be vaccinated becomes less important”, emphasizes Pascal Crépey.

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In other words, extending vaccination to the entire adult population provides the same advantages – limiting the number of deaths or hospitalizations – as if we vaccinate the people most at risk as a priority. “Indeed, the vaccination of younger people, who have a lower risk of developing severe forms but play an important role in transmission, makes it possible to reduce the circulation of the virus and therefore indirectly protect the most vulnerable”, write the study authors. “If the fact that vaccines have a real impact on the susceptibility to be contaminated is confirmed, we could think about opening up vaccination more quickly to the youngest”, says Pascal Crépey. This would make it possible to achieve the objective of 90% of the adult population protected more quickly.

But even if this threshold is not reached in the fall, “we will still reap the benefits of the vaccination campaign, because each person thus protected reduces the speed of spread of the virus”, underlines Jean-Stéphane Dhersin. The authorities should, according to him, slowly but surely be able to lift the constraints or adapt them. “We can imagine a reopening of places of culture, but where the public must wear a mask, for example,” he explains. It will just take a while to redefine the concept of “normal life”.


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