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In the race to develop a vaccine for bird flu, Vietnam has been a dark horse with early success. Vietnamese scientists have produced a prototype vaccine for the H5N1 avian-influenza strain and are planning human testing in August—just a few months behind top researchers in the U.S. There's good reason for the haste: 70% of the world's bird-flu deaths in the last two years occurred in Vietnam, and the government worries that the country could someday be ground zero of a pandemic if the flu mutates to become easily transferred among humans.

But the World Health Organization (WHO) and other scientists are worried about Vietnam's vaccine, which they say could itself make people sick, or even set off a pandemic. The problem is that the virus reference seed—the weakened bit of live H5N1 used to build up immunity in the human body—was mixed with cancer cells to help it replicate and then grown in a monkey kidney. That method is highly unorthodox. "People could get cancer from the vaccine," says Klaus Stohr, head of the WHO's global influenza program. Even more ominous, the developers say they've followed international procedures to ensure that the virus hasn't mutated in the making of the vaccine, but they haven't opened all their records or allowed an inspection of their labs. The chances of mutations are slim, says Robin Robinson, an epidemiologist and influenza expert at the U.S. Department of Health and Human Services, but the Vietnamese method "may have provided a means for emergence of mutated H5N1 viruses in humans that may lead to a pandemic."

WHO officials thought they had convinced Vietnam's government to call off human testing on its vaccine and develop a new one based on an approved virus seed provided by the WHO. But two top Vietnamese scientists tell TIME they will forge ahead with their own strain. "Nothing has changed," says Dr. Nguyen Thu Van, the head of the vaccine team. "We will test our vaccine on humans as planned before." There's little anyone can do: the WHO has no enforcement powers. "The danger is very unlikely," admits Michael Perdue, a WHO virus expert who has consulted with Vietnam. "But you just don't want to play with fire."

• Kay Johnson
• The WHO says Vietnam's bird flu vaccine program could hurt more than it helps