By Kate Kelland, Health and Science Correspondent
LONDON (Reuters) - Human cases of a deadly new strain of bird flu that has killed 27 people in China are likely to crop up in Europe and around the world but that should not cause undue alarm, Europe's leading flu expert said on Thursday.
In his first media interview since returning from an international scientific mission to China last week, Professor Angus Nicoll said the H7N9 flu outbreak in humans was one that should be taken extremely seriously and watched closely.
"We are at the start of a very long haul with H7N9," Nicoll told Reuters in a telephone interview from the Stockholm-based European Centre for Disease Prevention and Control (ECDC), where he is head of the influenza and respiratory viruses program.
He said there were many scientific questions to be answered about the new flu strain, which was first detected in patients in China in March having been previously unknown in humans.
The flu has so far infected at least 127 people in China and killed 27 of them, according to latest data from Chinese health authorities and the World Health Organization.
Scientific studies of the virus have established it is being transmitted from birds - probably mostly chickens - to people, making it a so-called zoonotic disease that humans catch from animals rather than from other humans.
Nicoll, who visited Beijing and Shanghai last week with a team of international scientific experts, confirmed what the WHO has repeatedly said - that there is no evidence yet of the virus efficiently passing from person to person - a factor that would make H7N9 a serious pandemic flu threat if it were to evolve.
Nicoll said the "most pressing public health question" for now was to identify the source of the circulating virus - the so-called "reservoir" - that is leading to chickens contracting it and sporadically passing it on to humans. This is likely to take time, with any results unlikely for several months.
In the meantime, Nicoll said the ECDC, which monitors disease in the European Union, and health authorities around the world should expect that "imported cases" of H7N9 flu may well begin to crop up elsewhere.
Just as Taiwan reported its first case on April 24, other countries should expect that business travelers and tourists may occasionally return from China having picked up the infection, he said.
"I'm not sure when that will happen. But the case in Taiwan shows that it can. If that person had got on a different flight and ended up in Paris, then we would have had a scenario that we would expect people to be alarmed at," he said.
"But again we should stress that this thing doesn't seem to be transmissible from human to human, so if we get some sporadic cases appearing in Europe, that doesn't change anything."
Nicoll noted that genetic analysis studies of H7N9 samples taken from patients in China showed the virus had already acquired two genetic mutations that made it more likely to be able to become transmissible between people.
Flu experts speaking at a briefing in London on Wednesday said those mutations, together with evidence that H7N9 is still mutating rapidly and probably spreading almost invisibly among birds because it does not make them obviously sick, meant this new flu was a "serious threat" to world health.
"You can never predict anything about flu, but it is concerning to see those mutations there, Nicoll said. "That's why it's important Europe should take this very seriously."
Nicoll added that he thought the Chinese were doing an "impressive job" handling, reporting, investigating and seeking to contain the outbreak.
(Editing by Alison Williams)