flu in humans has evolved into two strains, study finds
March 22, 2006
The H5N1 type of bird flu that affects humans has evolved into two
strains, American researchers have reported.
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
"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
The US Health and Human Services Department has
approved the development of a second H5N1 vaccine based on the
Several companies are working on
vaccines experimentally, although their formulations are not
expected to protect well, if at all, against any pandemic
A vaccine against a pandemic strain would have to be
based on the virus being passed from one person to
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.