(Adds expert comment, background on virus composition)
By Sui-Lee Wee and Kate Kelland
BEIJING/LONDON, April 24 (Reuters) - A new bird flu strain that has killed 22 people in China is "one of the most lethal" of its kind and transmits more easily to humans than another strain that has killed hundreds since 2003, a World Health Organization (WHO) expert said on Wednesday.
The H7N9 flu has infected 108 people in China since it was first detected in March, according to the Geneva-based WHO.
Although it is not clear exactly how people are being infected, experts say they see no evidence so far of the most worrisome scenario - sustained transmission between people.
An international team of scientists led by the WHO and the Chinese government conducted a five-day investigation in China, but said they were no closer to determining whether the virus might become transmissible between people.
"The situation remains complex and difficult and evolving," said Keiji Fukuda, the WHO's assistant director-general for health security.
"When we look at influenza viruses, this is an unusually dangerous virus for humans," he said at a briefing.
Another bird flu strain - H5N1 - has killed 30 of the 45 people it infected in China between 2003 and 2013, and although the H7N9 strain in the current outbreak has a lower fatality rate to date, Fukuda said: "This is definitely one of the most lethal influenza viruses that we've seen so far."
Scientists who have analysed genetic sequence data from samples from three H7N9 victims say the strain is a so-called "triple reassortant" virus with a mixture of genes from three other flu strains found in birds in Asia.
Recent pandemic viruses, including the H1N1 "swine flu" of 2009/2010, have been mixtures of mammal and bird flu - hybrids that are more likely to be milder because mammalian flu tends to make people less severely ill than bird flu.
Pure bird flu strains, such as the new H7N9 strain and the H5N1 flu, which has killed about 371 of 622 the people it has infected since 2003, are generally more deadly for people.
The team of experts, who began their investigation in China last week, said one problem in tracking H7N9 is the absence of visible illness in poultry.
Fukuda stressed that the team is still at the beginning of its investigation, and said that "we may just be seeing the most serious infections" at this point.
Based on the evidence, "this virus is more easily transmissible from poultry to humans than H5N1", he said.
Besides the initial cases of H7N9 in and around Shanghai, others have been detected in Beijing and five provinces. On Wednesday, Taiwan's Health Department said a businessman had contracted H7N9 while travelling in China and was in a serious condition in hospital.
Samples from chickens, ducks and pigeons from poultry markets have tested positive for H7N9, but those from migratory birds have not, suggesting that "the likely source of infection is poultry", said Nancy Cox, director of the influenza division at the U.S. Centers for Disease Control and Prevention.
John Oxford, a flu virologist at Queen Mary University of London, said the emergence of human H7N9 infections - a completely new strain in people - was "very, very unsettling".
"This virus seems to have been quietly spreading in chickens without anyone knowing about it," he told Reuters in London.
Flu experts say it is likely that more cases of human infection with H7N9 flu will emerge in the coming weeks and months, at least until the source of infection has been completely confirmed and effectively controlled.
Anne Kelso, the Melbourne-based director of the WHO Collaborating Centre for Reference and Research on Influenza said there has been a "dramatic slowdown of cases" in the commercial capital of Shanghai, which has recorded most of the deaths, something she described as "encouraging".
After Shanghai closed down its live poultry markets in early April, there was an almost immediate decline in new H7N9 cases, she said. "The evidence suggests that the closing of the live poultry markets was an effective way to reduce the risks."
Even so, the WHO's China representative, Michael O'Leary, issued figures last week showing that half of the patients analysed had no known contact with poultry. (Editing by Louise Ireland)