The H5N1 bird flu in humans has evolved into two separate strains, a development that will complicate the search for a vaccine and the prevention of a pandemic, US researchers have reported. The genetic diversification of the pool of H5N1 avian influenza viruses with the potential to cause a human influenza pandemic heightens the need for careful surveillance, researchers said at the International Conference on Emerging Infectious Diseases in Atlanta. "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 Rebecca Garten of the US Centres for Disease Control and Prevention, who helped conduct the study. One of the two strains, or clades, made people sick in Vietnam, Cambodia and Thailand in 2003 and 2004 and the second, a cousin of the first, caused the disease in people in Indonesia in 2005. Two clades may share the same ancestor but are genetically distinct - as are different clades, or strains, of the AIDS virus, the team from the CDC found. "This does complicate vaccine development. But we are moving very swiftly to develop vaccines against this new group of viruses," said Dr Nancy Cox, chief of the CDC's influenza branch.