Paris (AFP)
Fighting Covid-19 means looking to the future by creating vaccines, but also looking back to try to trace the origin of this new disease, one year after its emergence in China.
A meticulous quest, with an uncertain outcome.
- When does the Covid-19 date?
We can't say for sure.
All we know is that the first cases were spotted in the Chinese city of Wuhan a year ago, before the disease spread to Asia and then the world.
"It is believed that the first cases in Wuhan date from the beginning of December," said WHO (World Health Organization) on its website.
"But the place where an epidemic is first detected is not necessarily where it started", adds the WHO, according to which "we cannot exclude that the virus has circulated elsewhere, silently".
In recent months, researchers from different countries have ensured that cases had gone unnoticed long before December 2019, based on analyzes of wastewater or post-test blood samples.
But these claims have "not been confirmed", nuance Etienne Simon-Lorière, head of the unit of evolutionary genomics of RNA viruses at the Institut Pasteur (Paris).
To establish the genealogical tree of the virus, researchers rely in particular on genetics.
These analyzes "make it possible to better understand the dynamics of transmission, in particular the way in which the virus could have evolved over time and how several foci of infection can be linked together", according to the WHO.
The latter has set up an international team whose mission is to trace the origins of the virus.
She will have to investigate both the suspect animals and how the first patients may have been infected.
- How did the virus come about?
Scientists agree that the disease has an animal origin.
"The big question mark is what led to the transfer to Man," Etienne Simon-Lorière told AFP.
In all likelihood, the animal causing the virus is the bat, "a major reservoir for coronaviruses", he adds.
The intermediate animal remains to be known, since the SARS-CoV-2 virus is unlikely to have passed directly from bats to humans.
The pangolin was designated as a suspect very early on, on the basis of genetic analyzes.
But we are not sure that it is really him.
The WHO mission will need to clarify this point, by investigating other animals sold at the Huanan market in Wuhan, to which most of the early cases were linked.
"It may be that the intermediary is an animal which has a receptor for the virus very similar to that which we see in humans," notes Etienne Simon-Lorière.
This receptor is a protein called ACE2, which the virus attaches to to enter cells.
However, the mustelidae family (mink, ferrets, etc.) has a receptor very similar to that of humans, which is not the case for other animals.
However, nothing says that we will find the answer.
"The introduction of a new virus into the human population is one of the greatest mysteries that an epidemiologist can hope to unravel", underlines the WHO.
"It's a very random treasure hunt. For Ebola, we never managed to find it," adds Etienne Simon-Lorière.
- What about the laboratory?
In the spring, amid diplomatic tensions, US President Donald Trump claimed that the virus had accidentally escaped from a specialized laboratory in Wuhan.
China had rejected these accusations, but the debate is recurring.
"As long as we have not found the intermediate host, this hypothesis of an accidental escape cannot be ruled out," recently estimated virologist Etienne Decroly on the site of the French research center CNRS.
"It is a scenario that we are obliged to list, even if it seems implausible in practice, because that would imply a lot of secrecy and lies", judge Etienne Simon-Lorière.
On the other hand, the scientific community excludes another hypothesis brandished by conspiratorial circles, according to which the coronavirus would have been manufactured in the laboratory.
"All the elements of its genome have already been observed in nature, mainly in bat coronaviruses. There is therefore no indication that it could have been made by humans", according to Etienne Simon-Lorière .
- What timeframes for the WHO mission?
After sending a reconnaissance team in July, the WHO hopes that its team will be able "soon" to go on the ground in China, she said on November 23.
But science is not everything, and diplomatic considerations come into play.
The United States accused Beijing of hiding things and the WHO of bending too much to the will of the Chinese authorities.
Other WHO member states, while being less critical, suspect Beijing of slowing down the process.
"We want to know the origin and we will do everything to know it", promised Monday the boss of the WHO, Tedros Adhanom Ghebreyesus.
"There is nothing to hide," he said, lamenting that some have "politicized" this "technical question".
- Why does origin matter?
"Understanding how an epidemic started is essential to prevent further introductions of the virus into the human population", underlines the WHO.
The goal is to "put in place measures to avoid the emergence of a new SARS-CoV-3, 4, etc.", comments Etienne Simon-Lorière.
These measures would consist of "modifying our ways of being in contact with animals from which emergence is possible", for example by prohibiting certain types of breeding.
During the SARS epidemic in 2002, the ban on the consumption of civets and the closure of farms had helped prevent the reintroduction of the virus into humans.
This small mammal consumed in China had been identified as the intermediate host of the coronavirus responsible for this epidemic.
© 2020 AFP