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Bird flu in humans has evolved into two strains, study finds
March 22, 2006

Atlanta: The H5N1 type of bird flu that affects humans has evolved into two strains, American researchers have reported.

This could complicate attempts to develop a vaccine.

One strain, or clade, infected people in Vietnam, Cambodia and Thailand in 2003 and the second, people in Indonesia the following year. Two clades may have the same ancestor but are distinct - as are different clades of HIV - the team at the US Centres for Disease Control and Prevention has found.

"In 2003 we had one genetically distinct population of H5N1 with the potential to cause a human pandemic. Now we have two," said Rebecca Garten, a member of the study team.

Speaking here at the International Conference on Emerging Infectious Diseases, she said the pool of H5N1 candidates with the potential to cause a human pandemic was becoming more genetically diverse as it spread geographically.

The H5N1 strain has spread in Europe, Africa and parts of Asia. Since 2003, it has infected about 180 people, claiming the lives of about 100 of these.

The US Health and Human Services Department has approved the development of a second H5N1 vaccine based on the second clade.


Several companies are working on vaccines experimentally, although their formulations are not expected to protect well, if at all, against any pandemic strain.

A vaccine against a pandemic strain would have to be based on the virus being passed from one person to another.

Garten and her colleagues analysed more than 300 H5N1 virus samples taken from infected birds and people from 2003 until the middle of last year.

Most of the viruses, including those in all the human cases, were of the genotype Z. Now there are two clades of the Z genotype.
There were also small numbers of viruses in birds that were genotype V or W or the recently identified G.

Pakistan has become the latest country to confirm bird flu in poultry, while in Egypt a woman is believed to be infected, the third case in less than a week.

A man in Egypt has recovered after being given Tamiflu, but a woman who received the drug died on Friday.

- Reuters


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