A hospital team in France retrospectively tested a sample from someone hospitalised in December, and it came back positive for COVID-19.
COVID-19 may have been in France in late December last year — around a month earlier than previously thought.
Doctors at a hospital in the northern suburbs of Paris retested samples from patients between December 2, 2019, and January 16, 2020.
Of the 14 patient samples retested, one sample, from a 42-year-old man hospitalised on December 27, 2019, came back positive.
"The patient presented clinical signs and radiological patterns frequently seen previously in the Chinese and Italian cohorts," the study, published in the International Journal of Antimicrobial Agents, said.
Respiratory samples are frozen at -80°C and kept for four years in case they need to be further tested but the study's limitations included a lack of medical history records and the potential for a false-positive test.
In an interview with French media, the head of the intensive care unit where the patient was hospitalised, Dr Yves Cohen, said that inconclusive samples from other hospitalisations should also be retested.
The World Health Organization (WHO) echoed this sentiment stating that more cases could be found if countries retest samples from patients sick in December and January.
The findings "help to better understand the potential virus circulation of COVID-19," WHO spokesman Christian Lindmeier said in a video statement.
Lindmeier said it was not a surprising finding "given the earliest cases of COVID-19 had symptoms onset already in early December, so it's possible that some of these infected people travelled from Wuhan to other countries at that time".
But some experts have warned that the positive test could have been due to laboratory contamination.
The arrival of COVID-19 in Europe
A Spanish genome study estimated the virus first arrived in the country around February 14. In Italy, the first confirmed case dates back to the end of February.
The first confirmed case of COVID-19 in France was recorded on January 24: a Frenchman of Chinese origin and two Chinese tourists who had travelled to Wuhan.
A recent Institut Pasteur study found that one of the clades or groups of COVID-19 circulating in France does not match known cases imported from China or Italy.
The study analysed 97 genomes of the coronavirus that causes COVID-19 in France, 3 genomes in Algeria and 338 other sequences to study the initial introductions of the virus in France.
They concluded that "the virus was silently circulating in France in February, a scenario compatible with the large proportion of mild or asymptomatic diseases".