Four months later, the truth comes out. It is a retrospective work of analysis of the tests carried out on patients with pneumonia in December and January, which allowed Professor Yves Cohen, to announce this Sunday, on BFM TV, the detection of a positive case at Covid -19 December 27. But how is such an a posteriori discovery possible?
In this case, the teams of Professor Cohen, head of the intensive care unit at the Avicenne hospital in Bobigny (Seine-Saint-Denis) and Jean-Verdier, in Bondy, started with a proposal by Jean-Ralph Zahar, professor of hygiene in this last establishment. The two specialists decided to re-examine the files of 14 patients who had been tested in order to check if they were then carrying the flu or a coronavirus. All were negative.
At the time, doctors had performed PCR tests, where a swab was placed in the patient's nose to collect secretions. These swabs were again analyzed for a specific search this time for SARS-COV-2. And this time, one case turned out to be positive.
The same principle as for anti-doping laboratories
To achieve this discovery, the scientists benefited from an essential practice: the preservation by freezing of the samples. In both hospitals, they are kept at -80 ° C for four years.
"If you have a well-preserved sample, the analysis can be done months later," confirms Vincent Maréchal, professor of virology at the Sorbonne. In this case, it is the same principle that allows anti-doping laboratories to keep samples of athletes for testing several years later in the light of new methods of detecting certain products.
To find a posteriori trace of the Covid-19, there are today two methods. First the serological tests, also called indirect tests, which concern the detection of antibodies in the blood. "Depending on the case, they appear a few days or a few weeks after the clinical signs," explains Vincent Maréchal. Subjects who have been infected without clinical signs also develop antibodies, in quantity and within deadlines which remain to be confirmed. This test indicates that you have been infected, but it does not say when. "
The other method is based on virological tests, also called direct tests. "Currently, the benchmark test is RT-PCR, a very sensitive technique that makes it possible to search for the genome of the virus, that is to say its genetic information," continues the professor of virology. In general, the virus is sought in the upper respiratory tract by swabbing the nose or even the throat, and possibly in the lower respiratory tract via the bronchoalveolar fluid, which is less easy. "
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"Studies also show that the virus can be detected in the blood and especially in the stool, where the signals can be positive for several weeks," continues Vincent Maréchal. Well beyond the period of presence in the upper respiratory tract. "
"China should do the same"
Any sample kept by a hospital may therefore prove to be useful for a posteriori detection of Covid-19. "Provided that any risk of contamination or false positive is ruled out," however specifies Vincent Maréchal. In the official publication of their work, Yves Cohen and Jean-Ralph Zahar recognize on this subject that their study "has several limits". But to avoid any false positive results, they took care to use two different techniques and personnel.
One thing is therefore certain: this retroactive testing process should be encouraged. "If we have the samples, we must actually go back as far as possible in time to trace the natural history of the epidemic," emphasizes Vincent Maréchal. Analysis of the genetic sequences of the virus is also a priority to see how the circulation was made. China, the cradle of the pandemic, should obviously do the same. "