Covid-19: the number of people who have been in contact with the virus would be underestimated

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In an editorial published in the British Medical Journal on September 3, a statistician, immunologist and clinical researcher estimate that the number of people who have been in contact with the coronavirus and who are immune is underestimated.

In an editorial published September 3 in the British Medical Journal, a British biostatistician, clinical researcher and immunologist suggests that the proportion of the population that has already faced the coronavirus, and therefore is immune, may be underestimated. .

Serological tests would be very imperfect

"Current antibody tests cannot identify people who had mild infections," say the authors of the analysis. Serological tests, which aim to find out if the individual has recently been infected and therefore has developed antibodies against the coronavirus, would be very imperfect, according to specialists, who here speak of the seroprevalence of Sars-CoV-2.

"Seroprevalence studies use antibodies as markers of pathogen exposure to estimate the proportion of the population that has been infected with the coronavirus," the authors write. However, "considerable variations have been observed in the results of the SARS-CoV-2 seroprevalence studies," they add.

"Seroepidemiological studies may underestimate the true seroprevalence of Sars-CoV-2 for a number of reasons. Accuracy requires the use of a test sensitive enough to reliably detect antibody responses to mild infection in different post-infection settings. exposure, "say the experts. And in fact, serological tests have mostly been done on people who have developed a severe form of Covid-19. People with mild or even asymptomatic disease generally have not undergone serologic testing.

Antibodies sought, a bias of serological tests?

The other major bias, according to the authors, is related to the antibodies sought, as the choice of antibody would influence the performance of the test. "Among the tests authorized by the FDA (Food Drug Administration, American health authority), most detect only IgG and IgM (immunoglobulin) antibodies, the dominant components of the immune response carried by the blood. But IgA also plays an important role in the immune response to respiratory tract infections and appears immunologically relevant in Covid-19, especially in asymptomatic people, "the researchers add.

Therefore, by looking for only IgG and IgM antibodies, a past infection could be missed. Especially since IgA antibodies against coronavirus have already been detected in various biological samples such as serum, saliva or breast milk, the authors write. These antibodies would also be detectable earlier than IgG and IgM and would persist longer. The three Britons specifically cite a study from Luxembourg, published in the journal MedRxiv (and therefore pending scientific validation), which indicates that 11% of the 1,862 people taken would have IgA antibodies against the coronavirus, compared to 1.9% than what is in the IgG antibody.

The authors believe that current seroprevalence studies cannot detect people who have had a mild form of Covid-19. In fact, a greater proportion of the population than is believed may have encountered the virus and developed an immune response against it. From there to saying that herd immunity is achieved, there is a big step that no one is in a hurry to take.

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