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By Maggie Fox

Getting repeated flu vaccines year after year doesn’t reduce their effectiveness in children and, in fact, may boost immunity against some influenza strains, researchers reported Friday.

Their findings, published in the Journal of the American Medical Association, should reassure parents and doctors worried after reports suggested getting vaccines every year might reduce immunity.

The new study of more than 3,000 children found that kids who were vaccinated every year were less likely to get flu than unvaccinated kids. The multiple vaccinations tended to boost protection against one common flu strain: influenza B.

“In no case was repeated vaccination associated with lower effectiveness than vaccination in the current season only,” Sarah Cobey, who studies the body’s immune response to germs at the University of Chicago, wrote in a commentary.

“In other words, there was no evidence of diminished vaccine effectiveness in frequent vaccinees, even though the study included seasons in which such effects had been reported elsewhere,” added Cobey, who was not involved in the study.

Medical leaders recommend that nearly everyone 6 months and older get a new flu shot every year. That’s because the circulating flu strains vary from year to year, they mutate constantly, and because the effects of the vaccine wear off pretty quickly.

Flu vaccines are not as effective as most vaccines, which provide near-universal protection against infection and whose protection lasts for decades. Flu vaccines do, however, reduce the severity of disease and prevent death from influenza.

There have been questions about whether people really need a fresh flu shot every year. A few small studies, including one out of Canada, have suggested that getting the flu vaccine yearly might increase some people’s risk of catching the virus, even though it's difficult to explain why that might happen.

Huong McLean of the Marshfield Clinic Research Institute in Wisconsin has been studying flu vaccine efficacy for years. She and colleagues compared kids vaccinated two or three times to kids vaccinated for the first time to see if prior vaccination affected their risk of getting flu.

They did a detailed breakdown, comparing several flu seasons, comparing different types of vaccines, and checking efficacy by flu strain.

Every year, several different strains of influenza circulate — sometimes at the same time, sometimes one after the other. So flu immunizations are cocktails of three to four different vaccines protecting against H1N1, H3N2 and influenza B strains.

There are also different formulations, including a live but weakened version of the virus in the needle-free Flumist nasal spray, and inactivated or partial flu viruses in the shots.

Overall, there was little difference among kids vaccinated for the first time and those vaccinated for one or two years previously, the team found.

“Prior-season vaccination was not associated with reduced vaccine effectiveness,” they wrote. “These findings support current recommendations for annual influenza vaccination of children.”

Earlier this year, a different team found that repeat vaccinations also protected the elderly.

Americans do not clamor for flu vaccines. On Thursday, the Centers for Disease Control and Prevention reported that just 37 percent of Americans overall got the flu vaccine last season which, as it turned out, was the deadliest influenza season in decades. Close to 80,000 people died of flu in 2017-2018, the CDC estimates.

Which is the best flu vaccine?

McLean’s team has a detailed breakdown. The flu shot was very effective, lowering the risk of infection with H1N1 flu by 72 percent for kids who had been vaccinated the season before, and by 67.5 percent for those who had skipped the previous year.

Flumist lowered the risk of H1N1 by between 8 percent and 47 percent, McLean said.

For H3N2, the shot lowered a child’s risk of serious infection by just under 40 percent for those who had received a shot the year before and by 23 percent who had not been vaccinated the year before.

Flumist worked better against H3N2 than it did against H1N1, the team found. Kids who had been vaccinated the year before got 50 percent protection from FluMist, but no protection against H3N2 if they had not been vaccinated before.

Researchers have found evidence that repeated vaccinations boost immunity, especially in children. 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.