"This gives a whole new picture on everything," WHO spokesman Christian Lindmeier told a UN briefing in Geneva, referring to the French report.
"The findings help to better understand the potential virus circulation of Covid-19," he added, saying other earlier cases could emerge after retesting samples.
A French hospital which retested old samples from pneumonia patients discovered that it treated a man who had Covid-19 as early as Dec. 27, nearly a month before the French government confirmed its first cases.
There is not currently any suggestion that the virus emerged from anywhere other than Wuhan, China. It is thought that, instead, the French case was somehow connected to a person who travelled from there in December before the virus had been identified or reported by China, the WHO spokesman said.
However, the French patient, a fishmonger, is not thought to have a direct link to China or a history of recent travel and experts say the French case requires more investigation.
Lindmeier encouraged other countries to check records for pneumonia cases of unspecified origin in late 2019, since this would give a "new and clearer picture" of the outbreak.
Asked about the origins of the virus in China, Lindmeier stressed that it was "really, really important" to explore this.
U.S. Secretary of State Mike Pompeo says his country has evidence that the new coronavirus emerged from a laboratory in Wuhan, although scientists have advised the WHO that it is of animal origin.
"That may need further missions or a mission (to China) so we are looking forward to this," Lindmeier said.
The WHO's top emergencies expert, Dr Mike Ryan, said on Monday that the body's chief had raised the issue of the origins of the virus "at the highest level" during a WHO mission to China in January.
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