Tests of the first U.S. bird flu vaccine show that it is only moderately effective. It requires higher doses and immunizes a smaller percentage of people than vaccines for regular seasonal flu. Health officials hope to improve it for use if avian flu mutates to a form that can cause a human pandemic.
Research on 450 people shows that two injections of a new H5N1 bird flu vaccine raise immunity only in slightly over half of the adult volunteers inoculated with the highest dosage. Data in The New England Journal of Medicine reveal that the dose needed to accomplish this was 12 times higher than that needed for the annual seasonal flu shot. The percentage of people protected got smaller as the size of the dose decreased.
The chief of U.S. government infectious disease research, Dr. Anthony Fauci, says he had hoped the highest dose would protect at least 75 percent of the volunteers against avian flu. But he says the news is not all bad.
"There's some good news in that we know we have a vaccine that can induce an immune response, albeit at very high doses in the majority of people," he explained. "We certainly would like it to be higher than that. I think it's a bit of muted good news in that we are going in the right direction, but the sobering issue is that we have a long way to go."
In a New England Journal of Medicine editorial, Dr. Gregory Poland of the Mayo Clinic in Rochester, Minnesota says even a moderately effective bird flu vaccine could be useful by preventing or decreasing transmissibility, severe symptoms, complications, and even death.
The physician who led the study at three sites across the United States, John Treanor of the University of Rochester, New York, says the tests provide a road map to go forward.
"Every journey starts with the first step," he said. "Now we know that it's going to be a long road. We are much farther ahead than we would be if we had not started."
The bird flu vaccine is based on a virus strain that killed a Vietnamese man in 2004.
Because the necessary dosage is so high, it would be difficult to produce in quantity for a significant portion of the world's population using conventional methods. So researchers are turning their attention to ways to stretch the vaccine using adjuvants, substances designed to boost the immune system's response. If a study of 1,200 people across the United States shows that adjuvants enhance this vaccine, each person might be able to get one dose instead of two, making it available for more people.
But Dr. Fauci points out that, unlike seasonal influenza, most people have never had bird flu, so they don't have leftover immunity from the previous year, needing only a single booster to deal with a new variant strain. He says everyone might need the higher number of doses of a bird flu vaccine, just as an infant does the first time it gets a seasonal flu shot.
"It's hard to tell," he added. "It's going to depend on the response or how lucky we get with the adjuvants. It might require two, because it's truly a primary immunization as opposed to the secondary immunizations that we do with seasonal flu. We're going to assume we need two, but hopefully we can get away with one if the adjuvants work well."
Work is also under way on a vaccine based on a second bird flu strain that has already begun to kill people.