A new generation of vaccines may be necessary to effectively protect people from the flu and its complications, a new study indicates.

"We're stuck with a vaccine that has been around for 60 years and not changed much," said Michael Osterholm, an infectious disease specialist at the University of Minnesota.

The most common flu vaccine works for just 59 percent of healthy adults, the study found, well below the previously reported 70 to 90 percent.

Osterholm's team analyzed data from 31 studies over the past 40 years that tested for the presence of flu in lab tests, rather than measuring the increase in flu antibodies, which may overestimate the vaccine's effectiveness, researchers say.

Health officials now recommend that everyone over the age of 6 months get a flu shot. About 43 percent of the population got the vaccine last season, according to the Centers for Disease Control and Prevention.