Vaccinating chickens may be the only way out of the bird flu nightmare in Asia. But it could also lead to the evolution of new strains, the latest research shows, increasing the risk of a human pandemic. Only intensive surveillance can stop this happening, but experts say the countries affected do not have the necessary systems in place. Last week China declared its bird flu outbreaks had ended. Health officials are vaccinating millions of the birds that escaped slaughter. Indonesia is also vaccinating, and other Asian countries hit by the H_5N_1 bird flu are considering the same strategy. But the H_5N_1 virus is almost certainly still circulating among the vaccinated birds, and the fear is that in this abnormal setting it may evolve into a form that is not only fatal to people, like the current one, but can also spread from person to person. Research in Mexico has shown for the first time that under these conditions bird flu evolves at an unprecedented rate, with unpredictable consequences.
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