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 found.
"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 increased surveillance.
"As the virus continues its geographic expansion, it is also undergoing genetic diversity expansion," Garten said in a statement.
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 no exception.
"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 person.
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.
