Genetic analyses of samples from patients recently infected with the H5N1 bird flu virus suggest that new strains are emerging in the north of Vietnam.
The report, posted on the World Health Organization (WHO) website last week, says the data are limited and that more studies are needed, but cautions against complacency in the face of the growing pandemic threat (see Time to prepare for bird flu pandemic 'running out').
At a WHO meeting in the Philippines in early May, scientists also concluded that the transmission of the virus from person to person could be more common than previously thought.
The data — from the Centers for Disease Control and Prevention in Atlanta, United States, and Japan's National Institute of Infectious Diseases — could signify changes in the virus because clusters of infection are larger and more numerous than seen recently, and in some cases exposure to poultry could not be traced.
Meanwhile, cases of bird flu infection in people, pigs and poultry continue to be reported in China, Indonesia and Vietnam.