Annual flu shots protect only about 59 percent of the population -- far less than previously thought -- according to a new study led by University of Minnesota researchers.
The study found "major holes and gaps'' in the vaccine given to tens of millions of Americans every year to prevent influenza and its complications, said Michael Osterholm, the lead scientist and head of the university's Center for Infectious Disease Research and Policy. But until a better vaccine comes along, Osterholm said, 59 percent "is a lot better than zero."
In the past, experts have estimated that annual flu shots protect 70 to 90 percent of the population. But the new analysis, reviewing studies over four decades, found that on average, the vaccine worked on about 59 percent of adults younger than 65.
In fact, flu vaccines may have had little, if any, effect during some flu seasons, according to the report, which was published Tuesday. The most common flu shot, which uses an inactivated flu virus, had no noticeable impact in four out of 12 seasons studied. The nasal spray version, which is made with a live virus, had no apparent effect in three out of 12 seasons.
Osterholm said the studies don't explain why the vaccine's effectiveness varied so much. He said it did not depend on whether the vaccine, which is reformulated annually as flu strains change, was a close match to the viruses in circulation. "There's little rhyme or reason, any given year," he said.
Good and bad effects
Osterholm said he hopes the report will spur development of new and better generations of flu vaccines.
But some fear it could undercut support for existing vaccines.
"I certainly wouldn't want the public to think, 'Oh, well, this is just bad,'" said Kris Ehresmann, director of the infectious disease division at the Minnesota Department of Health.
"The vaccine we have has limitations. And this study says, yep, it's moderately effective, it's not perfect. But right now it's the only tool we have."
At the same time, experts need to pay attention to Osterholm's findings, said Ehresmann, who serves on the federal Advisory Committee on Immunization Practices.
"This is the most comprehensive review of influenza vaccine that I've seen," she said. She predicted it will be the subject of discussion this week in Atlanta, where committee members are meeting. "This study is sort of shaking the underpinnings, if you will, of that 70 to 90 percent [effectiveness] figure that's often been quoted."
Osterholm said he and his colleagues reviewed more than 5,700 studies published since 1967 and narrowed them down to 31 that met strict scientific criteria, to see how often flu infections occurred in people who had been vaccinated.
They found one encouraging note: The nasal spray vaccine was 83 percent effective in children age 7 or younger, the studies showed. But there was no proven benefit for older children or adults.
They also found that the H1N1 vaccine, which was developed during the 2009 flu pandemic, was 69 percent effective. In a more deadly pandemic, Osterholm said, that would have left far too many people unprotected, and it demonstrates "an urgent need for a new generation" of flu vaccines. "This is still largely a 1950s vaccine that is used today," he said.
There was little evidence, one way or the other, on people older than 65, because most studies excluded that age group. It's widely assumed that flu vaccine is less effective in older people, because immune systems weaken with age, according to Ehresmann. But health experts advise them to get flu shots anyway because they're considered at high risk for severe complications, such as pneumonia.
Osterholm said the research gives a more realistic view of the vaccine's weaknesses, but he hopes it doesn't prompt people to skip their flu shots.
"This is more like these vaccines are an iPhone 1.0. We need an iPhone 10.0." But until it's available, he said, "you don't throw the 1.0 away."
The study, which appears in the November issue of the Lancet Infectious Diseases, was published online Tuesday.
Maura Lerner • 612-673-7384