Paris (AFP)

This is one of the keys to unlocking the deconfinement lock: serology tests, which will tell if someone has had Covid-19 and can return to work, are one of the major international challenges for weeks to come.

"This serology, we all expect it, all over the world", assured the French Minister of Health, Olivier Véran.

"All the world research is focused" on these new tests, to "industrialize them as quickly as possible", he added, judging that it could take "from a few days to a few weeks at most".

"This is a major issue, especially in view of the deconfinement," insisted the French minister, whose country has been confined since mid-March.

The subject of a frantic race between biotechnology companies, "serological tests are being developed" and "are not yet evaluated," said the World Health Organization.

Based on a genetic analysis, current tests, called RT-PCR, allow to say that a patient is infected at the time when they are carried out.

Lighter (a blood test is enough), serology tests do not have the same objective: they aim to determine after the fact if an individual has been in contact with the virus, and if he is therefore a priori immune.

To do this, "they detect antibodies, that is to say the response that the immune system has brought" to defend themselves against the virus, explains Dr Andrew Preston, of the University of Bath (England).

These antibodies are of two types, he adds: "IgM (for immunoglobulins M), which are produced first" and can be detected "about a week after infection", then "IgG (immunoglobulins G), then produced ".

- "Licence" -

This ex-post analysis is all the more important since many people may have been infected without knowing it. Because in the case of the new coronavirus, it often happens that we develop little or no symptoms, while being contagious.

Already used for other diseases, the serology technique ("serum", liquid part of the blood plasma) can be performed automatically in any analysis laboratory, as soon as the tests specific to the new coronavirus have been assessed.

Once generalized, they can be used to determine who can break out of the confinement to which half of humanity is today subjected.

"The central question is how we secure everyone to get back to work," said AFP Dr. François Blanchecotte, president of the French Union of Biologists, who represents the analytical laboratories.

In Italy, the president of the Veneto region, Luca Zaia, has offered to grant workers a "license" after the serological test, to certify that they are not contagious.

And in Germany, "a sort of vaccination booklet for immunized people could emerge, which would allow them to resume their activities", explained to the newspaper Der Spiegel Gérard Krause, of the Center for Research on Infectious Diseases Helmholtz.

"A general practitioner told me yesterday that if I do a serological test and it is proven that he is immune, he will offer his services at the nearby hospital in a Covid unit without a belly ball," adds Dr. Philippe Hérent, managing director of Synlab Opale, a group of nine labs in the north of France.

After ordering 2,000 serological tests several weeks ago, he assured AFP that he had been delivered "from a small part of the order" by two suppliers, American and Italian, and was in the process of evaluating them.

However, he feared the risk of a shortage: "World demand is becoming enormous and production is limited".

- Mix -

But serology tests have several limitations.

"If they are used too soon", before the production of antibodies, "the patient could in fact still be carrying the virus and contagious," warns Dr. Michael Skinner, of Imperial College London.

This is why countries could decide to combine the two techniques, RT-PCR and serology, as recommended for example by Maurizio Sanguinetti, infectiologist at the Gemelli Foundation in Rome.

"Maybe it will be a mix of both: a diagnosis (by RT-PCR) to find out if you are still contagious and carrying the virus, and a serological test to find out if you have protective antibodies", adds Dr Blanchecotte .

The other pitfall, and not least, is that there is a lack of certainty about the immunity that one can acquire against the coronavirus.

"We cannot be 100% sure that a test that detects antibodies implies that the person is immune," warns Dr. Preston, even if this is the case "in the vast majority of infectious diseases".

More generally, the issue of serology tests goes beyond the question of containment. They will give "a much more precise picture of the extent of the epidemic in all countries", notes the WHO.

This picture is still very blurred, for two reasons: the impossibility of locating patients without symptoms and the large differences between the quantities of tests carried out by each country.

Serological tests will allow us to say what the proportion of the world population that has actually been infected and is therefore potentially immunized is, probably much higher than current estimates.

This will help to identify more precisely the mortality rate of the coronavirus, which is currently only the subject of estimates.

© 2020 AFP