WHO: Bird flu viruses diversifying, making vaccine target harder to pick

(Aug 19, 2006) The World Health Organization urged influenza vaccine makers Friday to use newer strains of virus when making vaccine to protect against H5N1 avian flu, saying the evolution of the microbe has led to increased variety in circulating strains.

While the diversity creates challenges for vaccine manufacturers - and potentially additional costs for the governments paying them to make and test vaccine against H5N1 - it does not mean the worrisome virus has moved closer to being able to spark a human flu pandemic, a senior official of the World Health Organization said.

"I don't think it's possible to interpret these kinds of changes in terms of whether the virus is moving closer to developing greater transmissibility properties among people," said Dr. Keiji Fukuda, co-ordinator of the WHO's global influenza program.

"I think what it simply reflects is that influenza viruses have evolutionary pressures on them and they evolve. They change. And that's what we're seeing."

Experts who have studied numerous samples of the virus have suggested nothing in them points to the development of mutations that would increase H5N1's ability to infect people and spread among them.

But an infectious diseases expert at the University of Minnesota questioned whether the scientific community knows enough about how influenza viruses adapt to a human host to be sure it would recognize it if H5N1 were on that path.

"What does it mean for an influenza strain today to be more adapted towards human-to-human transmission?" Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy, wondered.

He said he was not suggesting the virus is becoming more transmissible, only that the patterns that would lead to that end aren't defined.

With the spread of the virus to many different countries, sublineages of H5N1have emerged, each with distinct genetic properties. It's been known for at least a year that the viruses break down roughly into two families or "clades."

Clade 1 viruses have circulated in Cambodia, Thailand and Vietnam and caused human infections in those countries during 2004 and 2005. Clade 2 viruses circulated in birds in China and Indonesia in 2003 and 2004 and have since spread westward through the Middle East, Russia, Europe and Africa.

It is the clade 2 viruses that have caused most of the human H5N1 cases that have occurred since last 2005. And it is with representative samples from these viral strains that the WHO is now recommending vaccine makers work.

The Geneva-based agency isn't recommending work on vaccines made with the earlier strain - a virus isolated in Vietnam in early 2004 - be abandoned. At least a dozen companies are working on vaccine based on that seed strain.

Fukuda said there is an upside to working with additional virus types. The more practice manufacterers have working with the virus, the better placed they will be to move quickly to make a vaccine should H5N1 become a pandemic strain.

"To be able to work with more than one strain of H5 vaccine just gives the manufacturers that much of a leg up in terms of the experience that they may need later on," he said.

Since the current outbreak of H5N1 flu erupted in late 2003, at least 239 people have been infected with it and 140 of them have died. (from Canadian Press)


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