In a virology lab.
(Illustration) -
Fabien Dupoux / SIPA
In June and July, the Toulouse University Hospital laboratory carried out a serological test on more than 8,700 of its employees and followed the evolution at six months of the antibody levels of positive cases to the coronavirus.
According to a study published in the journal
Clinical Infectious Diseases
, the team from the Toulouse University Hospital virology laboratory shows that after being infected with Sars-Cov2 for the first time, there is an 84.8% chance of not having it. recontact within six months.
There are among the positive cases, 1.8% of cases of reinfection at six months according to this study.
If the natural protection thanks to the antibodies is important after a first infection, it is far from being as important as the 95% of immunity conferred according to the biostastician Chloé Diméglio, author of the study.
Are we protected once we have had the coronavirus?
Should we be vaccinated after a first infection?
Can we have Sars-Cov2 twice?
So many questions that we ask ourselves regularly and to which the study conducted by the virology laboratory of the Toulouse University Hospital, and published this week in the
Clinical Infectious Diseases
, provides answers.
In June and July, when the first wave is at the end of the race, the hospital decided to conduct a large survey with its caregivers and carried out 8,758 serological samples to find out the positivity rate.
“We had 276 positive for Sars-Cov2 in Elisa serology.
We put them to neutralization, we looked for the presence of neutralizing antibodies against Sars-Cov2.
This was the case, in over 95% of cases, ”explains Chloé Diméglio, biostatistician in the Toulouse University Hospital of Virology and lead author of the study.
After six months, antibody rising or stable
Among these cases, the team of scientists clearly does not establish any link between the presence of neutralizing antibodies and the fact of having developed a symptomatic infection or not.
No question of being satisfied with this photograph in the summer.
The researchers decided to follow these positive cases at six months to determine whether they retained these famous neutralizing antibodies long after their first infection.
They were therefore the subject of a new serological sample between December 30 and December 9.
“At the end of these 167 days of follow-up, the titer of neutralizing antibodies was for 96.7% of them either stable or increasing.
This means that the antibodies against Sars-Cov2 at six months are maintained, or even increase.
Which is very good news ”, pleads Chloé Diméglio.
1.8% re-infections
Of these 276 positives followed, there were five re-infections.
“It's not huge, it corresponds to a rate of 1.8%.
There was no link between the fact of having re-infected and the level of neutralizing antibodies, that is, there were re-infections with high and low rates.
There is also no link between the reinfection and the clinic, it is not because they developed a symptomatic infection the first time that they necessarily had an asymptomatic version the second, and vice versa ”, details the biostatistician.
But since there have only been five cases of re-infections, it is difficult for her to draw a scientific analysis.
The only tangible fact is that none of them was hospitalized, neither at the first nor at the second infection.
It is also difficult to determine if the reinfection is due to a variant, because the second time the viral loads were lower and the sequencing difficult to carry out.
84.8% chance of not being re-infected at six months
This 1.8% reinfection rate in people who had already contracted Sars-Cov2 in the first wave was compared with the infection rate of people who tested negative in June and July.
“We looked at whether these negative cases had meanwhile had PCR tests because they were symptomatic or contact cases.
We have established that there was an infection rate among these negative people, of 12.1%, ”says Chloé Diméglio.
To establish a protection rate, she therefore compared the infection rate in December of those who did not have Sars-Cov2 the first time and the rate of reinfection at the same time of positive cases this summer.
"This shows that having contracted a first infection with Sars-Cov2 induces protection of about 84.8%: when you contracted a first infection, you have an 84.8% chance of not having it. recontact ”, concludes the researcher.
Less protective than the vaccine
But if the antibodies developed during the first infection make it possible to have a rate of protection due to natural immunity of 84.8%, this remains lower than the rate of immunity provided by the vaccines.
“The immunity level conferred by vaccines is around 95% with Pfizer and Moderna.
There is therefore every interest in getting vaccinated even in the case of a first infection, beyond three months, ”argues the biostatiscian.
In the meantime, for the caregivers of the Toulouse University Hospital who caught a coronavirus infection during the first wave, and still remain very exposed today where the number of cases continues to increase, the good news is that the majority of them remain protected against a second infection.
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