Although health officials in the former colony are anxious not to cause panic, they are monitoring the new strain closely.

The infection, reported in New Scientist magazine, was identified in a 10-month-old girl admitted to Hong Kong's Tuen Mun hospital in late September.

A chicken flu strain caused panic

Although she was successfully treated, the virus bears all the molecular hallmarks of a porcine, or pig, strain.

This is a source of concern because research published in 1997 suggested that a virus which jumped from pigs to people was the source of the Spanish flu pandemic in 1918.

Alan Hay, director of the World Health Organisation influenza collaborating centre at the National Institute for Medical Research in London, said: "We're monitoring the case very carefully for that reason. We don't know the ins and outs of this yet."

New strains every year

New strains of flu emerge every year, which is why the standard influenza vaccination only remains effective for a short time.

Most are no more virulent than standard flu, but every few decades, a radically different virus emerges which can trigger pandemic infections killing millions across the globe - the last two were in 1957 and 1968.

Previous small outbreaks of swine influenza proved false alarms. In 1986, a Dutch man suffered severe pneumonia after contracting a virus, and in 1977 a similar virus turned up in a handful of people in Fort Dix, New Jersey.

Alan Kendal, who headed the Centre for Disease Control's influenza programme in the 1980s, said: "The question is: how do you not cry wolf while avoiding closing the door after the horse has bolted?"

Hong Kong is sensitive to the prospect of virulent flu strains crossing from the animal population.

Two years ago, a strain from chickens led to the deaths of six people, a third of those infected, and sparked a mass cull of chickens in a successful attempt to contain it.