Annual flu vaccines usually protect against the virus, but only for a while. Vaccines for other diseases typically work for years or decades. With the flu, though, next fall it will be time to get another dose. So why can't a flu vaccine last longer?

"In the history of vaccinology, it's the only one we update year to year," said Gary J. Nabel, the director of the Vaccine Research Center at the National Institute of Allergy and Infectious Diseases.

That has been the case ever since the flu vaccine was introduced in the 1950s. But a flurry of recent studies on the virus has brought some hope for long-lasting vaccines.

"That's the goal: two shots when you're young, and then boosters later in life. That's where we'd like to go," Dr. Nabel said.

Such a vaccine would help the fight against seasonal flu outbreaks, which kill an estimated 500,000 people a year.

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