The flu vaccine is less effective than widely thought, leaving too many people vulnerable to infection, especially if a particularly dangerous pandemic occurred, according to a new analysis.

Michael Osterholm of the University of Minnesota and his colleagues analyzed data from the highest-quality 31 studies published in medical journals between 1967 and 2011 and found that overall, the most commonly used flu vaccine has an efficacy rate of only about 59 percent for adults ages 18 to 65.

In contrast, the newer nasal spray flu vaccine is about 83 percent effective in children ages six months to seven years, the researchers reported in a paper published in the Lancet. The median rate of effectiveness of H1N1 vaccine was about 69 percent in people age 65 and younger, the researchers found.

“This amount of protection is not adequate for a pandemic setting,” the researchers wrote, adding that the findings underscore the need to develop more effective flu vaccines.

“The ongoing public health burden caused by seasonal influenza and the potential global impact of a severe pandemic really signals the urgent need for a new generation of highly effective and cross-protective vaccines that we can produce rapidly,” they wrote.

In the meantime, however, the researchers stressed that the existing vaccines are the best weapons available.

“We should maintain public support for present vaccines that are the best intervention available for seasonal influenza,” they wrote.

In a commentary accompanying the study, Heath Kelly from the Victorian Infectious Diseases Reference Laboratory in Australia and Marta Valenciano of EpiConcept in France said the findings indicate that ”now might be an appropriate time to use revised estimates of the most probably effectiveness of influenza vaccines to re-examine the effectiveness and cost-effectiveness of some policy options.”

To hear an interview with Osterholm, click here.