Paris (AFP)
Can we be protected against Covid-19 despite a negative serological test, or even without having ever caught this disease? A still poorly understood immunity, based on mechanisms other than antibodies, could perhaps
"Whatever the mechanisms, it is very likely that there are many people immunized without being detected by serology", that is to say by antibody tests, assures AFP Yonathan Freund, professor of emergency medicine at the Parisian Pitié-Salpêtrière hospital.
This theory rests on two pillars. On the one hand, the hypothesis of cross-immunity, that is to say the fact of being protected against the new coronavirus by having been infected in the past by other viruses of the same family, which cause common colds.
On the other hand, the still poorly understood role of T lymphocytes, a type of white blood cell responsible for the second part of the immune response (cellular immunity), the first coming from antibodies.
"The immune system is complex," German immunologist Andreas Thiel reminds AFP, co-author of a study published on July 29 in the medical journal Nature.
"Our main result is that at least a third of adults who had never been in contact with SARS-CoV-2 (the virus responsible for Covid-19, NDLR) had T lymphocytes capable of reacting to this virus. They most likely came from infections prior to other coronaviruses, "explains the researcher at Charité hospital in Berlin.
The work of a team from Singapore, made public on July 15 by Nature, had reached the same type of conclusion.
- Protection -
And this is also the case of another study, this one American, published Tuesday in the journal Science: in some individuals who have never been exposed to SARS-CoV-2, we find T lymphocytes which react to this new coronavirus as well as to four others, responsible for simple colds.
"This could help explain why some people with Covid-19 have few symptoms while others fall seriously ill," said one of the authors, Daniela Weiskopf, of the institute in a statement. La Jolla immunology in California.
This study builds on other work published in mid-May by the same team in the journal Cell. They showed the presence of T lymphocytes capable of reacting to SARS-CoV-2 in 40 to 60% of people who had never been exposed to this virus.
"More work is needed in several countries to assess whether these cells can provide immunity against SARS-CoV-2, or protection against severe forms of Covid-19", continues Andreas Thiel.
Because, whether or not we have been exposed to the new coronavirus, all this work points to the potential importance of T lymphocytes in the immune response against Covid-19, while we have until now focused a lot on antibodies .
According to a study by the Swedish hospital Karolinska in early July, Covid-19 patients with few or no symptoms could develop T-cell immunity, even if their antibody test is negative.
The vaccines currently in development seek to trigger both types of immune response.
- Under estimation? -
"If we have neutralizing antibodies, we are immune: it is the basis. But we must not consider that there is nothing else that exists", judge Yonathan Freund.
“Serology tests don't tell the whole truth, they tell it for a certain period only,” adds Andreas Thiel.
Studies have indeed shown that in patients affected by Covid-19, the level of antibodies could drop rapidly, within a few weeks.
"It can mean two things: one, which would be catastrophic, is that the immunity does not last in the Covid. But I do not think that is the case: out of 18 million cases, we do not has never heard of the slightest proven case of recurrence, ”comments Professor Freund.
“The other is that there may be people who are immune that cannot be detected by serology,” he adds.
Consequence: the immunity rate of the population, based on the detection of antibodies (or "seroprevalence"), could be underestimated all over the world.
"Seroprevalence studies give figures up to 15/20% in heavily affected areas. But we could be much more and some areas may have reached a threshold of immunity sufficient for there to be no more. disaster, "says Professor Freund.
"Understanding what impact the presence of these T lymphocytes in the population has on SARS-CoV-2 infection is of paramount importance for the management of the pandemic", write the authors of the German study.
However, it is only theory for the moment, emphasize the scientists. "It is much discussed, we are only on hypotheses and everyone is walking on eggshells", insists Professor Freund.
© 2020 AFP