Every evening, the Director General of Health, Professor Jérôme Salomon, announces at a press conference the number of new cases of Covid-19 identified since its previous point. And every day, the number of new people who test positive for SARS-CoV-2 exceeds the previous one. But what can we conclude from these figures?
The number of cases detected daily depends on the strategies and the screening capacity of each country. Logically, the more tests we carry out, the more cases of the disease are identified that would otherwise have gone unnoticed. Data from many countries confirm this quite well.
However, France, unlike other countries that have conducted very large screening campaigns, has chosen to test only severe cases of the disease, that is to say those most often causing respiratory complications serious and potentially fatal. A strategy consistent with the limited screening capacities available to it.
“We do not pretend to identify all cases”, confirms Daniel Lévy-Bruhl, epidemiologist and head of the Respiratory Infections Unit at Public Health France. “In Europe, I don’t know of any country that can identify all the chains of transmission. From stage 3, you have to accept that you can no longer identify cases individually. “
Other countries are testing more
Gold, if there is consensus on the usefulness of early detection, not all countries have limited testing to severe forms of Covid-19. In South Korea and Germany, massive screening campaigns have been or are still being implemented. Faced with an epidemic due to the Middle East respiratory syndrome coronavirus (MERS-CoV) South Korea emerged in 2015 during which 38 of the 186 infected people died, and has since strengthened its health surveillance system, which has enabled it to certify the first test kits early on. when the Covid-19 appeared (February 4, two weeks before the epidemic accelerated) and raised its capacity to 20,000 daily tests.
The Germans too quickly increased their screening capacity to 160,000 tests per week, according to Lothar Wieler, president of the Robert Koch Institute. Tests, reimbursed by social security, are offered in many places in the country. This explains both the high number of cases detected in each of these two countries and their particularly low case fatality rate (1% in South Korea and 0.3% in Germany, compared to 4% globally and 2.2 % in France), since most of the cases identified are mild forms of the disease.
According to the French approach, testing more people would above all amount to identifying mild cases, which do not require hospitalization or heavy medical care. And the success with which South Korea has fought the epidemic is not necessarily due to their screening strategy, according to Mr. Lévy-Bruhl:
“South Korea has implemented extremely drastic measures; the culture and the relationship to health threats are different. Of course, the very broad testing strategy contributed to this, but that is not the only explanation. “
Other factors, such as advanced surveillance capabilities (location of telephones, video surveillance, tracing of bank transactions), have indeed been cited to explain how effectively South Korea has managed the epidemic. However, the Korean testing organization is cited as a model by the World Health Organization (WHO), which urges affected countries to perform more tests.
Monday, March 16, the president of the organization, Tedros Adhanom Ghebreyesus, renewed these recommendations : “ Test, test, test, test all suspected cases of Covid-19. (…) You cannot fight a fire blindfolded. And we cannot stop this pandemic if we do not know who is infected. “
In France, increasing but limited screening
Is the same strategy applicable in France? Hard to say. Because if Mr. Lévy-Bruhl refutes the idea that the French strategy has come up against the limited logistical capacities of screening – these moreover are increasing day by day – force is to observe that these capacities are more limited than in other countries.
In France, only level 2 biological safety medical laboratories (PSM2 certified) are authorized to carry out these tests, in addition to reference health establishments, according to a decree issued on March 7. The number of laboratories mobilized is constantly evolving, assured the World the Director General of Health (DGS), Jérôme Salomon, during his press conference of March 19, without however communicating a precise figure. The DGS had indicated March 12 at 20 minutes that this represented 70 laboratories.
Currently, eight test kits are certified by the French authorities and distributed by six companies: Eurobio, Atothis, Orgentec, BD, Elitech and Roche. However, the number of these kits is limited and forces the health authorities to restrict their use to cases deemed to be priority, according to the recommendations issued by the High Authority for Health (March 6) and the High Council of Public Health (March 10). These priority cases include:
- patients hospitalized in intensive care for severe breathing difficulties;
- patients hospitalized for pneumonia with suggestive symptoms;
- health workers with suggestive symptoms;
- people at risk (already suffering from a pathology) and symptomatic pregnant women;
- patients hospitalized for another cause but who have become symptomatic.
Tests can also be carried out for organ or tissue donors, to monitor the condition of patients already admitted to intensive care or to investigate a possible epidemic focus in a community of people (three tests by Ehpad are for example authorized) .
In the end, although it may be interesting to compare the number of deaths due to the disease in each country to estimate the real extent of the epidemic, only a posteriori survey could give a precise estimate of it. Blood tests would then detect the antibodies produced against the virus and therefore identify people who had been infected.
How does the test detect the virus?
The tests that are performed in France are based on the detection of the genetic material of the virus in the body. To do this, a biologist or nurse first takes a sample from the nasal cavities using a swab (a kind of cotton swab).
The sample is then sent to a laboratory for testing by a method called “polymerase chain reaction” (or RT-PCR), which involves detecting whether RNA, the genetic material of the virus, is in the sample.
As such a small amount is difficult to detect, the test consists in adding two enzymes to the sample: the first (reverse transcriptase, called RT) will synthesize strands of DNA from the RNA of the virus. The second, DNA polymerase, will replicate this strand of DNA millions of times in a matter of hours. It then becomes possible to detect the presence of this DNA in the sample and thus to demonstrate the presence of the virus.
Our selection of articles on the coronavirus