Question asked by Céline on 09/09/2020
You ask us whether it is true that former infected people can present a negative serological test. We now know that the vast majority of people infected with Sars-Cov-2 produce specific antibodies against the virus. These antibodies (IgM and IgG) are detectable using serological tests. But let’s point out right away that the minimum amount of neutralizing antibodies necessary to be protected against reinfection is not yet known.
“With serological tests, you test the antibodies produced by the body against the virus, this is adaptive humoral immunity, one of the three components of the immune response. Since this immunity is adaptive, it takes some time. Between five and six days. It is detectable from around the seventh day, but it is recommended to measure it from the fourteenth day to be sure to spot everyone, because the production of antibodies varies enormously from one individual to another ”, explains Cédric Carbonneil, head of the evaluation service for professional acts of the Haute Autorité de santé. He also recalls that, for the time being, the serological test is considered “A diagnostic supplement”, for which the RT-PCR test is preferred. This type of examination should therefore not be carried out too soon after contamination, at the risk of obtaining a false negative.
“Decrease in neutralizers within two to four months”
As for the lifespan of antibodies in the blood, it varies greatly depending on the virus. For measles, for example, neutralizing antibodies (which help fight infection) are produced throughout life after infection or vaccination, compared with a few months for influenza. “From the outset, we had some concerns with the Sars-Cov-2, because with its cousins the Sars-Cov-1 and the Mers, we had immunity from a few months to a few years. Furthermore, coronaviruses and those responsible for rhinitis and winter viral infections do not provide long-term immune protection ”, continues Cédric Carbonneil.
So there was more chance that the amount of antibody would decrease rapidly. However, several studies published since the start of the pandemic only reinforce these fears. “To date, we know that the Sars-Cov-2 infection is capable of inducing the production of neutralizing antibodies in most patients. We are starting to have some controversial elements regarding the potential term of protection. Several studies have shown a decrease in neutralizers two to four months after infection. An article indicates that there is a drop in antibodies after two months. Another more recent indicated that the lifespan of neutralizing antibodies would be two months and disappearance after four months. There was also the case of a re-infected patient. The prevailing global hypothesis is that we would still be producing antibodies in the fairly short term ”, always indicates the head of HAS.
“Reactivated antibody production”
In France, the Institut Pasteur and the Strasbourg University Hospital are studying the evolution of the level of antibodies from former contaminants. A first study of 160 hospital staff was published in July 2020 and another is still ongoing. “In the first study, we knew that from the date of the first symptoms, in a period between twenty and forty days, we still saw a phase of increase. Antibody production corresponds to a bell-shaped curve. We studied the first phase of growth, there we study the second ”, explains Olivier Schwartz, head of the virus and immunity unit at the Institut Pasteur.
According to the preliminary results of the second study, the amount of antibody decreases on average by 50% after three months. “We have observed a drop in anti-Sars-Cov-2 antibodies, or even a complete negativation of the serology. This means that in these people, the antibodies are in very low quantities, and that they become undetectable by certain tests. On the other hand, this does not mean that they are no longer protected, because when they encounter the virus again, the body should reactivate the production of antibodies ”, explains Samira Fafi-Kremer, director of the Strasbourg Institute of Virology. A negative serological test would therefore not necessarily mean that the person is no longer protected.
“Antibodies are produced by cells [les lymphocytes B et T, ndlr] which can activate or reactivate and “produce” again in the event of another encounter with the virus, but this remains for a limited time. And this is the concern: from an antibody response towards one microbe to that towards another microbe, the durations of this memory vary enormously ”, summarizes Frédéric Altare, immunologist and research director at the National Institute of Health (Inserm). This is called cellular immunity. Another aspect of the immune response whose effectiveness against Sars-Cov-2, still unknown, and which does not appear in the serological tests currently carried out.