US study defines two clear bird flu strains
ATLANTA (Reuters) – The H5N1 strain of bird flu in humans
has evolved into two separate strains, U.S. researchers
reported on Monday, which could complicate developing a vaccine
and preventing a pandemic.
One strain, or clade, made people sick in Vietnam, Cambodia
and Thailand in 2003 and 2004 and a second, a cousin of the
first, caused the disease in people in Indonesia in 2004.
Two clades may share the same ancestor but are distinct —
as are different clades, or strains, of the AIDS virus, the
team from the U.S. Centers for Disease Control and Prevention
“Back in 2003 we only had one genetically distinct
population of H5N1 with the potential to cause a human
pandemic. Now we have two,” said the CDC’s Rebecca Garten, who
helped conduct the study.
Speaking to the International Conference on Emerging
Infectious Diseases in Atlanta, Garten said the pool of H5N1
candidates with the potential to cause a human influenza
pandemic is getting more genetically diverse, which makes
studying the virus more complex and heightens the need for
“As the virus continues its geographic expansion, it is
also undergoing genetic diversity expansion,” Garten said in a
The H5N1 strain of bird flu has spread across Europe,
Africa and parts of Asia and killed about 100 people worldwide
and infected about 180 since it re-emerged in 2003.
Although it is difficult to catch bird flu, people can
become infected if they come into close contact with infected
birds. Scientists fear the virus could mutate into a form that
could pass easily between humans, triggering a pandemic in
which millions could die.
All influenza viruses mutate easily, and H5N1 appears to be
“Only time will tell whether the virus evolves or mutates
in such a way that it can be transmitted from human to human
efficiently,” Garten said.
The U.S. Health and Human Services Department has already
recognized the two strains and approved the development of a
second H5N1 vaccine based on the second clade.
Several companies are working on H5N1 vaccines
experimentally, although current formulations are not expected
to protect very well, if at all, against any pandemic strain.
A vaccine against a pandemic flu strain would have to be
formulated using the actual virus passing from person to
For their study, Garten and colleagues analyzed more than
300 H5N1 virus samples taken from both infected birds and
people 2003 through the summer of 2005.
The majority of the viruses, including all the human cases,
belonged to 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 recently identified genotype G.