Paris (AFP)

Undetection, duration of isolation, tests ... The multiplication of cases of infection with the new coronavirus outside China raises questions about the surveillance systems implemented to prevent the spread of the epidemic worldwide .

- Under-detection?

To contain the epidemic, it is crucial to quickly identify and isolate sick people to prevent them from infecting others.

However, researchers at Imperial College London estimated Friday "that about two-thirds of the cases of Covid-19 out of China have remained undetected worldwide, with the potential result of multiple undetected chains of transmission human outside of China. "

In other work published Monday, an Anglo-American team estimates that "more than half of the infected people escape detection".

"One of the difficulties posed by this virus is the fact that there is (...) a whole spectrum of clinical manifestations", including mild forms, with few symptoms, explains to AFP Daniel Lévy- Bruhl, from the French health agency Santé publique France.

People with few symptoms can fall through the cracks.

Another category, even less detectable: that of patients who have no symptoms. Scientists believe, however, that their weight in the spread of the disease is undoubtedly limited.

"Perhaps (that these people) contribute a little bit, in any case certainly less than someone who sneezes, who coughs," comments Dr. Lévy-Bruhl.

"The investigations confirm that in the vast majority of cases, it is symptomatic people who transmit," he continues.

- How long for quarantine?

The incubation period, which separates the infection and the onset of symptoms, is estimated according to studies between two and 10 days. This led to the quarantine period of 14 days being set for suspicious cases.

But based on some cases, Chinese experts have estimated that the incubation period could be up to 24 or even 27 days. This would mean that a 14-day isolation could sometimes be insufficient.

This assumption leaves scientists skeptical for the time being.

"We never really believed it, the most recent data on the contrary go in the direction of a reduction in the duration of incubation", says Dr. Lévy-Bruhl. According to him, "there is very little chance that incubation times will go beyond" the 14 days.

And even if that were the case, it would remain "rare cases", underlines Professor Yazdan Yazdanpanah, French specialist and expert with the WHO. However, "an epidemic does not extend through extreme, very specific cases, it extends through a general profile", he continues. "Even if there are extreme cases, it is necessary to be interested in what is the most frequent".

- How effective are the tests?

Last week, two Australians and an Israeli passenger on the Diamond Princess liner tested positive for the new coronavirus when they returned to their country after they were suspected of having previously tested negative in Japan.

Their case raised questions but does not call into question the effectiveness of the tests: they could have simply escaped controls.

"The tests are very sensitive," said Professor Yazdanpanah. Based on the detection of the genetic code of the new coronavirus, these diagnostic tests must be carried out by specialized laboratories and the results are available within a few hours.

"At the beginning of the disease, people excrete a lot of virus and it is not a problem to detect them," according to Professor Yazdanpanah.

"Where it becomes more complicated, it is at the end of the illness: there, we can have tests which may not be positive but in these cases, in general, that also means that the patients are not very contagious (and) that they have no impact on the epidemic, "he adds.

- What consequences if the epidemic widens? -

The containment strategy put in place by the international community becomes more and more difficult to maintain as the number of affected countries increases.

If this strategy proves impossible to follow, the world health authorities will have to revise their ambitions downwards: instead of stemming the epidemic, each country will have to try to mitigate its effects.

"In this case, we can no longer have the same approach of monitoring all the patients nor of isolating all the contacts, because we no longer have the resources," notes Dr. Simon Cauchemez, of the Institute Pastor.

In developed countries, this can weigh on health systems.

"85% of infected people do not have a serious form of the disease, but minor forms," ​​said Professor Yazdanpanah. However, "severe forms are more severe than for the flu".

If the cases multiply, it will therefore "hospitalize people who have serious forms, without it being at the expense of everything else" in terms of hospital resources, he adds.

Obviously tenfold problems in poor countries where health systems are failing.

This is the whole paradox of Covid-19: significantly less deadly in absolute terms than a disease like Ebola, it is also more difficult to contain since cases are more difficult to spot.

"Even if only 3% of cases die, it can make significant figures if 30% or 60% of a population is infected," said Dr. Cauchemez.

© 2020 AFP