Some of the first doses of bird flu vaccine in the nation's stockpile are growing weaker with age. If the shots are needed anytime soon, there will be enough for a million fewer people than previously thought.

More up-to-date vaccine is being brewed to supplement the supply, which today has enough full-strength shots to vaccinate about 3 million people, according to an update issued this week by Health and Human Services Secretary Mike Leavitt.

Last summer, HHS officials were estimating that the stockpile had enough vaccine for 4 million people.

All vaccines lose potency if they spend enough time sitting on the shelf unused. Sure enough, routine testing uncovered that that has begun to happen with some of the first-brewed vaccine against the deadly Asian bird flu known as H5N1, HHS spokesman Bill Hall said Friday.

The government is stockpiling antiflu medications and a small amount of H5N1 vaccine in case the bird flu or some other super-strain sparks the next influenza pandemic. Here's the rub: If such a super-flu began circulating, it would take several months to begin brewing vaccine that was an exact genetic match. But the hope is that if H5N1 were the culprit, health workers and certain other people at high risk might get some protection from shots made against earlier strains of that virus.

The first batches in the nation's stockpile were brewed using an H5N1 strain that circulated in 2004. Now, manufacturers are brewing vaccine using a newer strain that circulated in Indonesia last year. With that updated version, HHS expects to have enough shots for another 5 million people sometime next year.

Meanwhile, the older shots' loss of strength is gradual, raising the question of whether some subpotent doses might be able to be used if absolutely necessary, Hall noted. "It doesn't go from 100 percent to zero," he said.