While the virus remains largely a bird disease and does not infect people easily, the scientists at a conference on avian flu and other infectious diseases in Singapore warned against any complacency.
"What's worrying is there were more (human) cases in 2006 than 2004 and 2005. The problem is still with us," Robert Webster of the St Jude Children's Research Hospital in the United States told Reuters on the sidelines of the conference.
"It's (H5N1) continuing to evolve and there are multiple lineages of this virus still out there. What cross-protection is there between these clades (strains) and sub-clades?"
Webster said several H5N1 strains had become widespread and different enough to cause unease among experts, and no one would dare assume that any one vaccine would be able to protect against other H5N1 strains.
"They are sufficiently different so that we're all making vaccine strains against each one of these -- we are making a large number of vaccine strains in case any one of these became dominant," he said.
H5N1 has killed 154 people since 2003 and experts fear it might trigger a pandemic if it learns to transmit easily among people.
Derek Smith of Cambridge University in Britain said there have been at least five major changes to the H5N1 virus since it was first discovered in 1959.
These five strains were found in Hong Kong in 1997, Vietnam in 2004, Eurasia-Africa in 2005-2006, Indonesia in 2005 and Anhui province in China in 2005.
"It's not clear what is driving this antigenic evolution," Smith, research associate at Cambridge's zoology department, told the conference.
Several companies around the world are in a race to develop vaccines against the virus, although many experts think they might not confer protection against an eventual pandemic strain, if one should emerge.
Viruses mutate constantly and since vaccines are made based on a selected strain, they might not work as well against other strains.
Some experts at the conference also urged governments in the developing world to be realistic. In the event of a pandemic, there simply would not be vaccines or drugs for poorer nations.
"The world's vaccine production capability is about 350 million doses of flu vaccine per year, so it's going to reach only a very small population of the world," said Roy Anderson of the department of infectious disease epidemiology at Imperial College in London.
More time and resources should be spent instead on researching about, and promoting, simple hygiene measures "that might be beneficial to the majority of the world at very low cost", he said, citing the use of surgical masks, alcohol sprays and regular hand-washing.
source - Reuters