PARIS — Weeks before Chinese authorities acknowledged that the coronavirus could be transmitted by humans, and nearly a month before the first officially recorded cases in Europe, a 42-year-old fishmonger showed up at a hospital in suburban Paris coughing, feverish and having trouble breathing. It was Dec 27.

Now doctors in France say that the December patient may have been the earliest known coronavirus case in Europe.

If confirmed, the case of the fishmonger, Amirouche Hammar, would mean the deadly virus made an appearance on the continent long before officials there began tackling it. Such a discovery would bring a strange new wrinkle to the story of the virus in Europe, one that has the potential of blowing up the previously established chronology.

The French government says it is looking at the report. The doctors who made the finding said they are confident in it and that they tested the patient’s old sample twice to avoid false positives. But they acknowledged that they could not completely rule out that possibility.

The doctors also cautioned that without further analysis of the sample, it was unclear whether the man had passed the virus on to anyone else, or whether his case was tied in any way to the epidemic that arrived later.

But if the timeline of when the virus appeared in Europe does change, the official efforts to combat the contagion will turn out to have been not just too late, but hopelessly too late.

By the time the first serious measures were put in place — the French government didn’t order a lockdown until March 16 — the virus may have already appeared three months earlier, according to a study of the new case that has been peer-reviewed and accepted for formal publication in the International Journal of Antimicrobial Agents.

That in turn would help explain the rapidly developing catastrophe that has since unfurled in France and Europe. There have been thousands of cases, hospitalizations and deaths, in numbers that only in recent weeks have begun to abate somewhat, as a result of the French government’s rigid confinement measures.

France alone has recorded more than 25,000 coronavirus deaths.

“If confirmed, what this case does highlight is the speed at which an infection starting in a seemingly remote part of the world, can quickly seed infections elsewhere,” said Professor Rowland Kao, the Sir Timothy O’Shea Professor of Veterinary Epidemiology and Data Science at the University of Edinburgh, in an interview for Britain’s Science Media Centre.

“Why is this important?” he added. “It means that the lead time we have for assessment and decision-making can be very short.”

The journal publishing the report about the case has had a brush with controversy over the coronavirus, walking back a study it published about treatments for the virus. And much about this apparent first case remains a mystery.

But the authors of the paper, doctors at the Avicenne Hospital in the Paris suburb of Bobigny, among others, declare flatly: Their study is of a “patient infected with Covid-19 one month before the first reported cases in our country” whose “lack of recent travel suggests that the disease was already spreading among the French population at the end of December 2019.”

French authorities declared the first official cases of coronavirus in the country — three people who had all recently been in China — on Jan 24. That was four days after China for the first time confirmed human-to-human transmission.

The doctors retested a sample from a patient who had suffered from pneumonia. They found the coronavirus.

“There’s no doubt for us,” said Dr Yves Cohen, head of intensive care at the Avicenne and Jean Verdier hospitals, in the northern suburbs of Paris, and one of the authors of the study, in a telephone interview Tuesday (May 5). “It was already there in December.”

What is not clear is how the patient, Mr Hammar, got it. Apart from a trip to Algeria last summer, he had not traveled. His wife, however, briefly exhibited some of the symptoms — coughing, principally — of the coronavirus, Dr Cohen said.

“We’ve got some theories,” he said. “His wife had a little cough.”

Mr Hammar’s wife, Fatiha, who works in a supermarket near Charles de Gaulle airport in Paris, told French television this week that she serves customers who come directly from the airport, “with their suitcases,” she said.

There were direct flights between that airport and the one in Wuhan, China, before borders were closed.

The first case outside of China was reported in Thailand on Jan 13. But experts have long suspected that the coronavirus may have spread internationally before the first officially reported cases.

The detection of the potential new case in France follows similar instances in the United States, where officials recently discovered that deaths from the virus had occurred weeks earlier than previously known, and a model suggested that silent outbreaks had spread for weeks before detection.

The French government has said very little about the case so far.

France’s health ministry said Tuesday that authorities were in contact with scientists and experts from other countries on the timeline of the spread of the virus, and that they would carry out further investigations “if they appear necessary.”

“We are in permanent contact with our European and Chinese counterparts on the issue, in order to better understand the spread of the virus at the global level,” the ministry said.

Amirouche Hammar, who lives in Bobigny, a northern suburb of Paris, said in an interview with BFM TV that he drove himself to the emergency ward at 5 am on Dec. 27 after several days of coughing, difficulty breathing and chest pains.

Mr Hammar, who has a history of asthma and diabetes, was diagnosed with a pulmonary infection but quickly recovered and was discharged two days later.

“I was surprised, given the devastation that the illness is causing,” Hammar said of learning, months later, that he had tested positive for Covid-19. THE NEW YORK TIMES