Does the Flu Shot Lose Effectiveness If You Get It Every Year?

There’s more evidence than ever that getting your flu shot—and getting it every year—is a good idea. A few studies had previously suggested that repeated vaccinations year after year might lessen the shot’s effectiveness, although most findings had been inconsistent and inconclusive. Now, a newer and larger study has found no evidence of such an effect. In fact, researchers say, getting vaccinated every year actually seems to increase immunity in some cases.

The new study, published last week in JAMA, included more than 3,000 children ages 2 to 17, all of whom visited the doctor for an acute respiratory illness between 2013 and 2016. All of the visits were during flu season, but only 23% of the patients tested positive for the flu.

When they looked at the patients’ immunization records, the researchers found that kids who’d been vaccinated during the current flu season, as well as the previous one, were no more likely to have been diagnosed with influenza than those who’d been vaccinated during the current season only.

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The effects varied by flu type (influenza A or B), by vaccine type (the nasal spray versus the flu shot), and by year. But overall, there was no scenario in which vaccination two years in a row was associated with lower effectiveness than vaccination in the current season only.

“In other words, there was no evidence of diminished vaccine effectiveness in frequent vaccines, even though the study included seasons in which such effects had been reported elsewhere,” wrote Sarah Cobey, PhD, associate professor of ecology and evolution at the University of Chicago, in a related commentary. The study is “important progress” in better understanding flu vaccine effectiveness, Cobey also wrote. 

In some cases, getting the flu vaccine every year seemed to actually increase protection. Children who’d received the FluMist nasal spray in the current season, for example, were less likely to catch the H3N2 strain of the flu (the most dangerous type to circulate last year) if they’d also been vaccinated the previous year.

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And for influenza B—one of two common flu types circulating seasonally in the United States—a prior year’s vaccination seemed to offer some residual immunity, even for kids who hadn’t been vaccinated again during the current season.

The researchers found similar results for additional analyses that included vaccinations in two and three prior seasons, as well.

Because the study was done in children, it’s not clear whether the same patterns would apply to adults. (However, a study published earlier this year in the Canadian Medical Association Journal did find that repeated vaccination is protective for people over 65.) And because every flu season is different—and every year’s vaccine has different levels of effectiveness—the study authors say there’s still a lot to learn.

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“To better understand the effect, if any, of repeat vaccination on vaccine effectiveness, additional studies are needed each season, for each vaccine type, in different populations,” Huong McLean, PhD, a research scientist at the Marshfield Clinic Health System and first author of the study, tells Health.

But for now, she says, the findings are reassuring—especially for parents who wonder if their children should be vaccinated every year. “Getting the flu shot every year is still the best way to protect against the flu,” McLean says.

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