Each sample was retested several times to ensure there were no errors, he added. Neither Cohen nor his team were immediately available for comment on Monday.

France, which has seen almost 25,000 people die from the virus since March 1, confirmed its first three COVID-19 cases on January 24, including two patients in Paris and another in the south-western city of Bordeaux.

Cohen said it was too early to know if the patient whose December 27 test was COVID-19 positive is France's "patient zero". Knowing who was the first is critical to understanding how the virus spread.

Cohen said the patient had survived and that a first investigation to trace the first contamination has been carried out.

"He was sick for 15 days and infected his two children, but not his wife, who works in a supermarket," Cohen said.

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"He was amazed, he didn't understand how he had been infected. We put the puzzle together and he had not made any trips. The only contact that he had was with his wife."

The man's wife worked alongside a sushi stand, close to colleagues of Chinese origin, Cohen said. It was not clear whether those colleagues had travelled to China, and the local health authority should investigate, he said.

"We're wondering whether she was asymptomatic," he said.

"He may be the 'patient zero', but perhaps there are others in other regions. All the negative PCRs for pneumonia must be tested again. The virus was probably circulating (then)," he said.

Reuters