The announcement that Chiron Corporation will not be allowed to distribute its flu vaccine is worrisome beyond the mere prospect of a shortage. That's because one scientist says those who had a flu shot last season, and don't get one this year, may be even more likely to get the flu.

"This is a known phenomenon," said bioengineer Michael Deem with Rice University. "It's called 'original antigenic sin'. And original antigenic sin is a description of the fact that if you get a flu shot this year, not next year, you may be more likely to get the flu next year."

A flu shot gives the immune system something to remember to fight off. But the flu virus mutates very rapidly, so the vaccine is changed every year.

In the journal Physical Review Letters, Deem reported that skipping that shot one year can mislead the immune system.

"The immune system may use its memory from last year and try to combat this year's flu with last year's antibodies," Deem said. "The memory in the immune system is actually leading the immune system astray."

An annual flu shot, Deem says, keeps the immune system up to speed.

"And this allows the immune system to build up its repertoire of antibodies that can control the flu for that particular individual," he said.

The Centers for Disease Control and Prevention targets flu shots to those at greater risk because of age or health. But if you had a flu shot last year, and don't get one this year, you may be more likely to catch this year's flu.

Because of the shortage, the CDC has changed its recommendations of who should get a flu-shot. For more information visit www.sciencentral.com