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explanations from Samuel Alizon, infectious disease specialist

As the state prepares to lift the containment of the population very gradually as of May 11, several questions are pending, including that related to collective immunity and that of being infected twice by the virus. Explanations from Samuel Alizon, CNRS researcher at the Infectious diseases and vectors laboratory: ecology, genetics, evolution and control.

To face the pandemic world of coronavirus ((SARS-CoV-2), the strategy of the French government has been the implementation of containment very strict since March 17, rather than betting onimmunity collective, with the objective of significantly reducing the circulation of virus in the population to try to stop its spread and avoid saturation of resuscitation services.

This strategy has " successful for the moment since theepidemic is under control Explains Samuel Alizon, CNRS researcher at the Infectious Diseases and Vectors laboratory. The base breeding number, R zero (R0), which indicates the " number of secondary infections caused by an infected person during duration infection in a fully susceptible population ", Went from 3.3 at the start of confinement to 0.5, says the Institut Pasteur in a study made public today. But, if the circulation of the virus is under control and tends to disappear, because of this confinement, the " French population is not yet sufficiently immunized "

If we trust theInstitut Pasteur study, it is expected that on May 11, only nearly 6% of French people should have been infected with SARS-CoV-2, with a higher proportion in Ile-de-France (12.3%) and in the Great East (11.8%) than in the Great West for example.

We will be " very far from the level of group immunity necessary to avoid a second wave if all containment measures were to be lifted " To avoid an upsurge in the circulation of the virus, or even a second wave, " without vaccine or treatment, it is estimated that 70% of the population must have been infected " This threshold necessary for " achieve group immunity and thereby prevent future epidemics ", obviously cannot be reached by May 11. If all measures to control the coronavirus were fully withdrawn by that date, the " risk of an upsurge in the number of people infected with the virus is great "

Limit the value of R zero below 1

The challenge is to limit the value of the basic reproduction number to 1, R zero. If this value stabilizes below 1, it is likely that " by the end of June, beginning of July, there will no longer be any new cases declared in France, apart from the cases imported from abroad " From May 11, it will be necessary to wait for a “ approximately two weeks before having reliable measurement indicators to measure the impact of deconfinement on the circulation of the virus in the French population "

As for the duration of this collective and natural immunity, it is " still under debate due to the lack of hindsight necessary to measure it over time " As a reminder, this virus was discovered only a few months ago, in December 2019. Although there is a consensus that in the short term this immunity will be effective, " beyond a year we have no certainty " As for the risks of being contaminated twice by SARS-CoV-2, they are " very weak in a short time interval " The absence of hindsight, " prevents us from having a clear vision on the future evolution of this virus beyond a few months " Having said that, the reports of the various national agencies do not make any case of people who “ after being cured of the virus would have again been contaminated ", Except in rare and very specific cases which" may can be explained by bad diagnoses (false positives, for example) or particular health conditions "

Can we get coronavirus twice?

ABSMARTHEALTH article with AFP-Relaxnews published on 03/19/2020

Many mysteries surround the disease of Covid-19. Can it be caught twice or is the body immune after being infected with SARS-Cov-2 and for how long? In this fight against the pandemic, the only possible answer seems to be the vaccine, and while deconfinement is being prepared, serological tests could turn into immunity passports, thus raising ethical questions.

Can we catch the Covid-19 twice? This crucial question in the fight against the pandemic today has no firm answer, even if scientists hope that an infected patient will be immunized against the new coronavirus at least for a few months. " Being immune means that you have developed a response immune against a virus that will allow you to eliminate it. And like the immune response has a memory, it also allows you not to be re-infected with the same virus later "Explains Eric Vivier, professor of immunology at the public assistance of the hospitals of Marseille.

Generally, for viruses with ARN like the Sars-Cov-2, you have to " about three weeks to have a sufficient amount ofantibody protectors " and this protection lasts several months, he continues. But that's the theory: the Sars-Cov-2, which keeps on keeping surprises, is too new to allow the least certainty. " We don't know, we can only extrapolate from other coronaviruses and even for them, data is limited "Said Mike Ryan, director of emergency programs at the World Health Organization (WHO).

Extrapolations from other coronaviruses

For SARS, which killed nearly 800 people worldwide in 2002-2003, the sick healed were protected " on average for two to three years ", Says AFP Professor François Balloux, University College from London. So, " we can certainly get re-infected, but the question is: after how long? We will only know this retroactively "

A recent Chinese study, not evaluated by other scientists, has certainly shown that rhesus macaques infected with this virus, and then cured, could not be reinfected. But that doesn't mean anything in terms of duration, says Frédéric Tangy, a researcher at the Institut Pasteur, because the observation took place over a relatively short period, namely one month.

In this context, reports from Asia, in particular from South Korea, reporting several patients who have been cured and tested again, raise many questions. In theory, it could be a second contamination, note several experts, who however consider this unlikely and favor other explanations at this stage.

In some people, the virus may not go away and infect " chronically "Like the virusherpes who can stay sleeping and asymptomatic, notes Professor Balloux. The tests not being 100% reliable, it could also be a false negative, since the patient has in fact never been rid of the virus. " It would suggest that people remain infectious for a long time, several weeks. It's not ideal ", He adds.

Too much uncertainty around protective antibodies

A study of 175 patients cured in Shanghai, published in early April without evaluation, shows that most of them developed neutralizing antibodies between 10 and 15 days after the onset of the disease, at various concentrations.

But " whether antibody means immunity is a different matter "Said Maria Van Kerkhove, another manager of the epidemic at WHO. " We are asking ourselves the question to know if someone who made a Covid (…) is so protected as that ", Worried Wednesday Professor Jean-François Delfraissy, president of the Scientific Council in France.

And worse: " We do not know if the antibodies we develop against the virus are not a risk of increasing the disease "Says Frédéric Tangy, noting that the symptoms the worst of Covid-19 comes late, when the patient has developed antibodies.

No evidence either for the moment to say who would develop more effective antibodies: sick most seriously affected or most spared, elderly or young …? Faced with these uncertainties, some question the relevance of reaching via contaminations collective immunity (when the epidemic ends for want of new people to be contaminated). " The only real solution is a vaccine ", Says Archie Clements, epidemiologist at Australian Curtin University.

Towards the creation of an immunity passport?

Despite everything, campaigns of serological tests (which detect antibodies) are launched to better know the part, probably very small, of the populations having been contaminated, as in Finland and in the United Kingdom. Or in Germany, where a research center even evokes a sort of "passport" of immunity allowing positive people to resume their activities.

" It's too premature, assures AFP Dr Saad Omer, director of Yale Institute for Global Health, which suggests waiting a few months for more reliable results. When there are serological tests sufficiently sensitive and specific ". The researchers indeed emphasize the need in particular for these tests not to be deceived by the antibodies of other benign coronaviruses in circulation.

But certificates also raise ethical questions, some researchers insist. And " people who need to work, to feed their families, could seek to be infected ", Warns Professor Balloux.

This will also interest you

Are we protected after SARS-CoV-2 infection? The re-emergence of cases already infected in South Korea raises fears of a weak immunity after an infection with SARS-CoV-2. So what do we know about the antibody response generated by our body? Overview of the subject in this ehealth with two experts.

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