Samples from 24 patients treated in December and January who had tested negative for flu before COVID-19 developed into a pandemic were retested by French researchers led by Yves Cohen, head of resuscitation at Avicenne and Jean Verdier hospitals;
The results, published in the International Journal of Antimicrobial Agents, showed that one patient-a 42-year-old man born in Algeria who had lived in France for many years and worked as a fishmonger-had been infected with COVID-19 "one month before our country's first reported cases," they said.
The results were "not surprising" the World Health Organization said.
"More early cases may also be found," WHO spokesman Christian Lindmeier told a U.N. Briefing from Geneva. In late 2019, he encouraged other countries to check records for cases, saying that this would give the world a 'new and clearer picture' of the outbreak.
Independent experts said more investigation was needed on the findings.
"It's not impossible for it to be an early introduction but the evidence is by no means conclusive," said Jonathan Ball, a professor of molecular virology at Nottingham University in Britain.
Stephen Griffin, an expert at the Institute of Medical Research at the University of Leeds, said it was "a potentially important finding" and added: "We have to be careful when interpreting these findings.'
Cohen told French television Monday it was too early to know if France's "patient zero" was the patient whose last trip to Algeria was in August 2019.
But "identifying the first infected patient is of great epidemiological interest as it dramatically changes our knowledge about SARS-COV-2 (the new coronavirus) and its spread in the country," he and his co-investigators wrote in the paper detailing their findings.
They said the absence of a link with China and the lack of recent travel "suggests that by the end of December 2019, the disease had already spread among the French population."
France, where nearly 25,000 people have died as a result of COVID-19 since 1 March, confirmed its first three cases on 24 January, including two patients in Paris and another in Bordeaux's southwest.
Rowland Kao, professor of veterinary epidemiology and data science at the University of Edinburgh, said that even if it was confirmed, the identification of a positive COVID-19 in December "is not necessarily an indication that the spread of COVID-19 from France began this early."
"If confirmed, what this case does highlight is the speed at which an infection can rapidly seed infections elsewhere starting in a seemingly distant part of the world," he said.