Avian-influenza virus is evolving, so no one can predict the exact genetic makeup of a killer bird-flu strain that would spread from person to person and cause a pandemic. So, if such a strain arose, manufacturers would be hard-pressed to rapidly make enough effective vaccine.

Scientists are looking for ways to stretch the amount of vaccine that would be available. One team now reports that priming people against bird flu with an existing, if not perfectly specific, vaccine might render a specially tailored one more potent during a pandemic.

Currently, three strains of the bird-flu virus, called H5N1, are known to infect people. Nega Ali Goji, a physician at the University of Rochester School of Medicine and Dentistry, and his colleagues identified 37 people who 8 years ago had received two doses of an experimental vaccine against one H5N1 strain of bird flu. That strain from Hong Kong was the first to jump the species barrier from birds to people.

In their study, Goji and his colleagues administered a recently developed bird-flu vaccine that targets an H5N1 strain identified in Vietnam in 2004. Each of the 37 volunteers from the earlier study received a single injection into muscle, as did 103 people who hadn't been previously vaccinated against bird flu.

Four weeks after the injection, the people primed with the earlier vaccine had made, on average, more than four times as many antibodies against the virus as the people in the other group had, says Goji, who presented the findings last week in Toronto at a meeting of the Infectious Diseases Society of America.

"If this is confirmed in larger studies, a prepandemic-vaccination program could be considered," Goji says.

"This was definitely a very positive finding," says Kathleen M. Neuzil, an infectious-disease physician at the University of Washington in Seattle. But she cautions that the single dose of new vaccine given to people in this study was quite large.

The finding suggests that with pre-vaccination, a smaller dose of an emergency vaccine might be sufficient, says physician and study coauthor John Treanor, also of the University of Rochester.

At the same meeting, physician Shital M. Patel of Baylor College of Medicine in Houston and her colleagues reported on another approach to economizing on bird-flu vaccine. They gave the new vaccine in three small doses, spread over several months, to 77 volunteers. Some doses were 1/30th the size of the shots that Goji and Treanor used. The vaccine was injected just under the skin, which can boost effectiveness (SN: 11/13/04, p. 307: http://www.sciencenews.org/articles/20041113/fob1.asp). Nonetheless, the low doses failed to consistently produce a robust immune response.

Meanwhile, other data show that the spread of bird flu among people could be catastrophic. Between December 2003 and September 2006, the World Health Organization confirmed 247 cases of bird flu in people. Jeffrey S. Markowitz of Health Data Analytics in Princeton Junction, N.J., reported at the Toronto meeting that 58 percent of those people died, compared with a fatality rate of 2.5 percent during the flu pandemic of 1918.

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