PARIS, April 29 (Xinhua) -- A genetic study conducted by scientists from France's Pasteur Institute suggests that the spread of the coronavirus epidemic in France is very little linked to cases imported from China or Italy, but to a locally circulating strain whose origin is unknown yet, reported French daily Les Echos on Wednesday.
In the study supervised by Sylvie van der Werf, director of the National Reference Center for Respiratory Viruses at the Pasteur Institute, and Etienne Simon-Loriere, virologist at the same institute, researchers discovered that the main strain of the coronavirus present in France was linked to a genetic group, or a clade, which had no link with China or Italy.
This study "has shaken up what we thought of the origins of this epidemic in France" and "illustrates the difficulty of containing it," commented Les Echos.
In this non-peer reviewed paper released on bioRxiv.org last week, researchers said they sequenced 97 severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) genomes from samples collected between Jan. 24 and March 24 from infected patients in France.
The first European cases sampled on Jan. 24 from the Ile-de-France region were directly imported ones from Hubei, China and the genomes fall accordingly near the base of the tree, within clade V, according to GISAID (a global database on influenza) nomenclature.
"Phylogenetic analysis identified several early independent SARS-CoV-2 introductions without local transmission, highlighting the efficacy of the measures taken to prevent virus spread from symptomatic cases," said the paper.
"In parallel, our genomic data reveals the later predominant circulation of a major clade in many French regions, and implies local circulation of the virus in undocumented infections prior to the wave of COVID-19 cases," it added.
All other sequences from Northern France fall in clade G, and this includes sequences captured during the steep increase of reported cases in many strongly affected regions, according to the paper.
"While a more thorough sampling will be needed to confirm this observation, it suggests that, unlike what is observed for many other European countries, the French outbreak has been mainly seeded by one or several variants of this clade," said the researchers.
Several genomes correspond to patients with recent history of travel in Europe, the United Arab Emirates, Madagascar or Egypt, and might represent additional introductions of the same clade.
"As the earliest representative of clade G had no history of travel or contact with returning travelers, we can infer that the virus was silently circulating in France in February, a scenario compatible with the large proportion of mild or asymptomatic diseases, and observations in other European countries," the paper added.
Unable to say where this strain comes from and how long it has been circulating in France without having been detected, the researchers believe that their work is sounding the alarm about the need to better monitor asymptomatic patients.
"This study emphasizes the importance continuous and geographically broad genomic sequencing and calls for further efforts with inclusion of asymptomatic infections," said the paper. Enditem