Since the start of the Covid-19 pandemic, an expression has often come up: “collective immunity”.
Seeking to reach it has been presented as an effective avenue in the face of the coronavirus.
This is the choice that some countries, notably Sweden, tried when the virus arrived last spring.
The principle seems simple (and promising): when a good part of the population has been infected by a virus, an epidemic goes out on its own.
But this strategy seems futile today.
This is evidenced by the low percentage of individuals having been infected in France and the concerns related to the rebound in the number of new cases in Stockholm.
“Never in the history of public health has collective immunity been used as a strategy to respond to an epidemic, let alone a pandemic.
It is scientifically and ethically problematic, ”even declared the director general of the World Health Organization, Tedros Adhanom Ghebreyesus, this Monday afternoon.
When is collective immunity achieved?
It all depends on the virus we are talking about.
The percentage of the infected population needed to achieve herd immunity depends on the initial reproduction number, called “R0”, of the disease.
This is the number of people that an infected individual will infect.
The mathematical formula then applied is simple: 1-1 / R0.
In the case of the SARS-CoV-2 coronavirus, this R0 is estimated at around 3. We then arrive at a ratio of around 0.67 (obtained by calculating 1-1 / 3).
This means that 67% of the population must be infected for herd immunity to be achieved.
How is the proportion of infected population calculated?
By researching the number of people who have developed an immune response, automatic in the event of infection by a virus.
There are several forms.
The most common are the production of antibodies in the blood or in the mucous cells of the airways.
“The inflammatory response modulates the immune response.
The more people have a symptomatic form of the disease, the more antibodies they generate, ”says Jean-Daniel Lelièvre, head of the infectious diseases department at Henri Mondor hospital in Créteil (Val-de-Marne).
The immune response can also go through T lymphocytes. Anyone infected, even if they have few symptoms, would produce such “cell killer” lymphocytes which make it possible to limit a new appearance of Covid-19, according to a study published in the journal Cell on August 14.
This would be especially true in patients with a severe form of the disease, discovered researchers from Inserm, the University of Tours and the CHRU of Tours at the end of September.
Where are we in France?
According to an "EpiCov" study carried out by Inserm and by the Directorate for Research, Studies, Evaluation and Statistics (DREES), the results of which were unveiled this Friday, October 9, 4.5% of the The French population had antibodies in their blood and therefore came into contact with the virus at the end of May, ie after the "first wave" of the epidemic.
This figure varies greatly depending on the region, from 1.5% in Bourgogne Franche-Comté to 9.2% in Île-de-France.
The researchers carried out a serological test, consisting of a simple blood sample, to 12,400 people representative of the national population.
"Based on the number of hospitalizations today compared to that at the time, it is estimated that this share of the population having been in contact with the virus is now around 7 or 7.5%" , advance with the Parisian Fabrice Carrat, professor of public health at Sorbonne University and coordinator of the Sapris survey, conducted in parallel with EpiCoV.
This rate is still increasing today, since around 20,000 people were newly positive every day at the start of last week (Monday to Thursday, these data being communicated on D + 3 by Public Health France).
But it is slowly increasing, since this figure of 20,000 must be compared to the population of 67 million inhabitants.
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We do not have such data for antibodies in the mucous membranes or for T lymphocytes. But even taking into account all forms of immunity, “we would remain very far [from the level of 67% or 70% of the population]. , especially since it would still be necessary to be certain that these people are always protected at the moment T, points out the immunologist Jean-Daniel Lelièvre.
Indeed, it is not yet clear how long antibodies (or T lymphocytes) can remain effective.
A few cases of reinfection with the SARS-Cov-2 coronavirus have also been documented around the world.
But they are extremely few at the moment and we must remain cautious about the causes of these isolated phenomena.
"I do not really believe in the possibility of reinfection, at least in the short term", estimates Fabrice Carrat, whose study is continuing on the same individuals in order, precisely, to "be able to certify that the antibodies protect in the event of new contamination ".
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To date, the fact that a natural infection protects "has not been demonstrated, in particular for subjects who have only been asymptomatic carriers", also warns the Institut Pasteur in a note updated on 9 September.
The current situation "does not allow the free circulation of the virus to be envisaged given the porosity between young people (20-40 years) and older people", also writes the Scientific Council in an "alert note" from September 22.
The risks of such a strategy
Anyway, betting on collective immunity to Covid-19 is damn risky in terms of the human toll.
"The cost of group immunity through natural infection would be very high, particularly in the absence of better patient care and without optimal protection of individuals at risk of serious complications," write researchers at the Institut Pasteur, Simon Cauchemez and Arnaud Fontanet, in a note published on September 9 in the journal Nature.
"With a collective immunity threshold of 50%, the toll would be 100,000 to 450,000 deaths in France," they estimate.
That is 3 to 14 times more than the total reached since the start of the pandemic (nearly 33,000 deaths).
"The impact in terms of hospitalizations and deaths in France would not be bearable", also judges Fabrice Carrat.
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Unsurprisingly, the government is on the same line.
“What we want is for the virus to circulate as little as possible.
Because behind there are lives, people who can find themselves in the hospital, ”summed up his spokesperson, Gabriel Attal, on September 21 on France Inter.
Sweden is worried
Without saying it openly, Sweden had made this bet to let the virus circulate in the hope of achieving collective immunity.
6000 deaths later, the ratio of deaths to the total population in the Nordic country is mid-October in the average of large European countries (including France) ... but far ahead of its neighbors.
Per capita mortality is indeed 5 to 10 times lower in Denmark, Finland and Norway, which were however affected by the first wave at around the same time but which have put in place much more restrictive containment measures.
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And the Stockholm region has seen a rebound in the number of infections since the start of the school year.
1,253 cases were recorded there in week 40, against 923, 537 and 305 the previous three weeks, according to official data.
A study published on August 12 in the scientific journal Journal of Royal Society of Medicine estimated that 15% of the population of Stockholm had developed antibodies, less than what authorities had hoped.
"If the Covid-19 had been predictable and behaved like other diseases, this rapid increase should now rather take place in Gothenburg, which was relatively spared in the spring and summer", admitted the epidemiologist "star" Anders Tegnell at the Göteborgs-Posten.
What about the vaccine in all of this?
Inevitably, if this coronavirus continues to circulate for months or even years, more than half of the population will eventually have been in contact with it.
But that's where the last form of immunity, unnatural this time, comes in: the vaccine.
This could make it possible to make a large part of the population "resistant" very quickly, without the risks of "classic" infection.
It is for this reason that all hopes are turning to the many laboratories launched in a veritable "global vaccine race".
Several of them are in phase 3 clinical trials, with the hope that a product will be available to the general public next year.