Does screening work when you are cured?

Q: “During a recent trip to Spain, I suffered from symptoms associated with COVID-19 (dry cough, very high fever, body aches and persistent sore throat). I consulted and, after a few days of medication, I got better. Back in Saguenay, I was tested on March 23 and my result was negative. I asked the nurse if I could still have COVID-19 and be cured of it. She couldn’t answer me. So does the test detect the virus if we are already cured? ” asks Mario Hubert of Laterrière.
A: The type of test currently used is called “PCR test”, for polymerase chain reaction. Essentially, it is a technique which “amplifies” a known genetic sequence, here a part of the genome of the microbe which one suspects the presence: if the sequence is absent, nothing happens; if it is present, then PCR makes millions of copies, which facilitates detection. So this test only detects the viruses that are present: if someone has recovered, their body no longer contains any virus, therefore no more viral genetic material, and the PCR will “see” nothing.

To find out if COVID-19 has already been done in the past, you need a test that detects not the virus itself, but antibodies, that is, particles that the immune system produces to deactivate a microbe. Antibodies are always very specialized particles: they only “stick” to one kind of microbe in particular, and not to others. In addition, the immune system must “learn” to make them one by one, disease by disease, which it can only do if it is confronted with the microbe itself. A person who has never been infected with COVID-19 will therefore not have antibodies specific to this virus. And because the antibodies stay in the blood for a long time after the patient is healed, they are a great way to find out if you have been infected in the past.

The problem, however, is that these tests are not yet available, at least not in Quebec. “It is much more complicated to develop than a PCR test,” explains Luigi Bouchard, professor of biochemistry at the University of Sherbrooke and head of the molecular and genetic engineering service at the CIUSS in Saguenay-Lac-Saint-Jean. It requires more research before having a validated and reliable test. From what I understand, it is coming, there are large groups working on it, but we are not quite there. ”

That said, things are moving very quickly, says Bouchard. The German company EuroImmun announced at the end of March that it had developed a test which detects antibodies and that the European Union approved it. This test or another equivalent should therefore soon arrive in Canada.


COVID-19 raises a lot of questions. In order to respond to as many people as possible, science journalists have decided to join forces. The media members of the National Cooperative of Independent Information ( Le Soleil, Le Droit, La Tribune, Le Nouvelliste, Le Quotidien and La Voix de l’Est ), Québec Science and the Center Déclic team up to answer your questions . You have some? Write to us . This project is made possible thanks to a contribution from the Chief Scientist of Quebec , who invites you to follow him on Facebook , Twitter and Instagram .

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