Science

Covid-19: Can antigenic self-tests be a game-changer?

Will antigenic self-tests change the face of the epidemic? Authorized since March 16 by the High Authority for Health (HAS), these devices have been sold in pharmacies to the entire population since April 12. If only three million tests have been delivered in pharmacies, according to the Directorate General of Health, the National Education has just ordered 64 million more, announced Prime Minister Jean Castex on April 22. The objective of the ministry is to screen the personnel of the National Education from May 3, then the high school students from the 10 and probably the less than 15 years old in the weeks which will follow, since the HAS recently authorized their use at the youngest.

But the self-tests raise several questions: how effective are they, are they more practical, do they not risk distorting the official figures of the number of new cases and can they be carried out in sufficient number so that their impact on screening is significant? The express takes stock.

How do they work?

The main advantage of the antigenic self-test is that it allows you to get a result in 15-20 minutes. In addition, it is known to be less invasive and less painful than nasopharyngeal PCR tests, since if both cases involve pushing a rod into the nasopharynx, the sample may be shallower with the self-tests.

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“The detection method is also changing,” specifies Bernard Binétruy, Inserm research director and scientific communication manager at the Marseille regional delegation. With PCR tests, we are looking for viral genomic sequences, that is to say its RNA code. With the self-tests, it is a question of detecting the antigens of the virus – the viral proteins -, those which are essentially found on the surface of the virus “. The sample is then placed in a tube containing strips that contain specific antibodies for Sars-CoV-2. If they react to viral proteins, the test is positive.

Are they effective?

Scientific data available to date indicates that antigenic self-tests are less sensitive than RT-PCR tests. In symptomatic patients, who usually have a greater amount of virus in the nasal cavities, the sensitivity is estimated at 81%. But in asymptomatic people, it varies between 50 and 58%. In other words, the risk of false positives is between 42 and 50% for tests on asymptomatic people. However, self-tests are a screening tool, not a diagnostic tool: they are intended for asymptomatic people. Symptomatic patients must perform RT-PCR tests, which are much more sensitive.

Should we therefore consider that the self-tests are in part unnecessary? “They remain useful, because even if we detect only 50% of asymptomatic, they are people who are carriers and do not necessarily know it and who would probably not have been detected otherwise, analyzes Bernard Binétruy. therefore better than nothing “. It remains difficult to assess the real usefulness of these tests, but epidemiologists believe that they could reduce transmissions, especially at school, especially because the detection of asymptomatic students will lead to more quickly closing classes and better block the spread of the virus. “Antigenic tests on a nasal sample appear relevant for breaking chains of contamination, especially in schools,” confirms the HAS.

Will the official figures of new positive cases be impacted?

The self-tests nevertheless raise a concern: some people may not report to Health Insurance that they are positive. Thus, statistics on the number of new positive cases, a relevant indicator for monitoring the epidemic, could be biased.

The hypothesis is nevertheless swept away by Bernard Binétruy: “These tests target asymptomatic people, who were not going to be tested anyway, did not necessarily know that they were contaminated, and who therefore already escaped the official figures. self-tests, some will surely continue to escape the statistics, but others will take the step to report and especially to perform a PCR test after a positive test [afin de confirmer le diagnostic et d’identifier le variant, NDLR]. The government has often been criticized for its lack of confidence in the population, but by setting up the self-test, it appeals to civic consciousness and especially that of the youngest: this is a way of saying that it is counting on them to do what is necessary to protect themselves and protect others “, wants to believe the specialist. It will also be necessary to count on the prudence of each one not to forget to maintain the gestures barriers in the event of negative test.

Will the youngest succeed in taking the tests?

Another subject of concern: the ability of citizens to carry out these tests themselves, and in particular of the youngest. “Students, high school and college students can carry out the self-test independently after a first performance under the supervision of a competent adult if necessary, puts the HAS into perspective. For students in primary school, the initially supervised self-test is also possible. but it is preferable that the test be carried out by the parents or the trained personnel. For children in nursery school, the sampling and the test must be carried out by these same actors “.

For his part, Jean-Michel Blanquer assured that tutorials will be offered to the staff of National Education in order to explain the smooth running of the procedure. “The high school students will be accompanied by competent, voluntary staff: both medical staff, doctors and nurses, who will have the support of other staff, for example, sometimes the laboratories that accompany us in this work and release staff, assures the minister. It is not something very complicated. ”

Will National Education have enough tests?

Theoretically, the order of 64 million self-tests should make it possible to practice more than six million weekly screenings until the end of the school year, since there is a little less than ten weeks before the summer vacation. And if we take into account the fact that around twelve million students attend schools, colleges and high schools and that around one million teachers and education personnel support them, a little less than 50% of the The whole could therefore be screened once a week by July 6.

In fact, the announcement was made late, on April 22, “which suggests that the government has not ordered these tests before,” said Mahmoud Zureik, professor of epidemiology and public health at the university. Versailles-Saint-Quentin-en-Yvelines and co-founder of the Collective “Du Côté de la Science”. No official figure has been communicated on the orders. “They will probably be staggered over time, advance Bernard Binétruy. But the government is probably counting on a real rise in power at the start of the school year, in September”.


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