As this year’s flu season gathers steam, doctors and pharmacists have a fresh stock of vaccines to offer their patients. The vaccines usually provide strong protection 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.

“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 a change. Dr. Nabel and other flu experts foresee a time when seasonal flu shots are a thing of the past, replaced by 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. He predicted that scientists would reach that goal before long — “in our lifetime, for sure, unless you’re 90 years old,” he said.