Here's a good reason to get a flu shot every single year: people who got a new shot every flu season were less likely to die or end up in the hospital with severe flu, a new study shows.
A look at people taken to the hospital with influenza showed that those who had been vaccinated repeatedly did better than those who had only been vaccinated in the current year.
The study of people over 65 — who are most likely to die from influenza — showed that repeated vaccination was twice as good at preventing the most severe complications of flu.
"The prevention of severe and fatal infection caused by influenza was observed mainly in patients who were vaccinated in both the current and previous seasons," Dr. Itziar Casado and Dr. Jesús Castilla of the Institute for Public Health in Pamplona, Spain, wrote in the Canadian Medical Association Journal.
"Repeated vaccination for influenza was highly effective in preventing severe and fatal infection caused by influenza in older adults," they added.
This makes sense to David Topham, a microbiologist and immunologist at the University of Rochester Medical Center, who was not involved in the research.
"We probably only get exposed to influenza, the virus, every three to five years on average," Topham told NBC News.
Related: Most kids who died of flu weren't vaccinated
"That's plenty of time for your immunity to wane. Getting the vaccine every year boosts your immunity."
And repeated vaccination may cause broader immunity, both Topham and the researchers said. Vaccines stimulate both the production of antibodies and cells called T-cells to fight germs. Repeated vaccination might more effectively boost the action of T-cells.
It's a reassuring finding. A couple of small studies had suggested that getting the flu vaccine every year might increase some people's risk of catching flu, even though it was hard to explain why that might happen. Topham said he did not think the evidence shows repeated vaccines are harmful.
"I think it might not be the case for most people," he said. "It was a very slight effect." And, he said, most studies had shown the opposite — that getting a flu vaccine every year prevents infection better than getting vaccinated once in a while.
A separate study suggested that pregnant women who got a certain flu vaccine two years in a row were more likely to miscarry,although again researchers said this conflicted with most of the other research that has been done.
Related:Studies Show Flu Vaccines Safe for Pregnant Women
U.S. officials recommend that just about everybody over the age of 6 months should get a flu vaccine every single year. The flu strains circulating often mutate and the vaccines are updated in an attempt to keep up.
This year the flu season is off to a strong start, with hospitals in some states already reporting their emergency rooms are filling up. So far, 13 children have died from flu.
Even if the flu shot does not completely protect against infection, the research shows that it greatly lowers the rate of people who die or get severely ill from flu.
Flu kills between 2,000 to 56,000 Americans every year, depending on how bad the flu season is, and it puts 140,000 to 710,000 people into the hospital.
For the study in Spain, the team checked on 728 people 65 and older who were in the hospital with flu. Of these patients, 83 died.
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Patients who had not been vaccinated at all were more likely to need intensive care — 16 percent of them did, compared to 6 percent of those who had been vaccinated. And 14 percent of those who died had not been vaccinated, compared to 6 percent who had been vaccinated.
The vaccine was 70 percent effective in preventing death.
"Vaccination that occurred only in the current season did not show significant protection against severe influenza," the researchers noted. But repeated vaccination did.
It's possible that people who get vaccines every year are healthier and more careful in other ways, Topham said. For instance, one study showed people who get flu vaccines are also less likely to have car accidents, he said.
"There is very little downside to getting your influenza vaccine," said Topham. "I get my vaccine every year. Everyone in my family does."