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By Maggie Fox
The repeated flu vaccines, year after year, do not reduce their effectiveness in children and, in fact, could enhance immunity against certain strains of influenza, researchers reported Friday.
Their findings, published in the Journal of the American Medical Association, should reassure concerned parents and physicians after information suggests that receiving a vaccine each year could reduce immunity.
The new study of more than 3,000 children revealed that children vaccinated each year were less likely to get the flu than unvaccinated children. Multiple vaccinations tended to enhance protection against a common strain of influenza: influenza B.
"In no case has repeated vaccination been associated with lower efficacy than vaccination for the current season alone," wrote Sarah Cobey, who studies the body's immune response to germs at the University of Chicago .
"In other words, there was no evidence of diminished vaccine efficacy in frequent vaccinated patients, although the study included seasons in which such effects had been reported." elsewhere, "added Cobey, who did not participate in the study.
Medical officials recommend that almost everyone aged 6 months and older be vaccinated each year. Because circulating influenza strains vary from year to year, they constantly mutate and the effects of the vaccine subside rather quickly.
Influenza vaccines are not as effective as most vaccines, which provide almost universal protection against infection and protection for decades. Influenza vaccines, however, reduce the severity of illness and prevent deaths from influenza.
We wondered if people really needed a new flu shot every year. A few small studies, including one abroad, have suggested that the annual influenza vaccine increases the risk of contracting the virus in some people, although it is difficult to explain why this might happen.
Huong McLean, of the Marshfield Clinic's Research Institute, Wisconsin, has been studying the effectiveness of the influenza vaccine for years. She and her colleagues compared vaccinated children two or three times to those vaccinated for the first time to see if previous vaccination had affected their risk of getting the flu.
They carried out a detailed breakdown comparing several influenza seasons, comparing different types of vaccines and checking the effectiveness by influenza strain.
Each year, several different influenza strains circulate – sometimes at the same time, sometimes one after the other. Influenza vaccines are therefore cocktails of three to four different vaccines protecting against the H1N1, H3N2 and B strains.
There are also different formulations, including a live but weakened version of the virus in Flumist No Needle Nasal Spray and Inactivated or Partial Influenza Viruses in Injections.
Overall, the team found that there was little difference between children vaccinated for the first time and those vaccinated for one or two years.
"Vaccination from the previous season has not been associated with reduced efficacy of the vaccine," they wrote. "These results support the current recommendations regarding annual flu vaccination for children."
Earlier this year, a different team discovered that repeated vaccinations also protect older people.
Americans do not call for flu shots. Thursday, the Centers for Disease Control and Prevention reported that only 37% of Americans had been vaccinated against influenza last season, which was the deadliest season in decades. Nearly 80,000 people died of the flu in 2017-18, according to CDC estimates.
What is the best flu vaccine?
The McLean team presents a detailed breakdown. The influenza vaccine was very effective, reducing the risk of H1N1 flu infection by 72% for children immunized the previous season and 67.5% for those who had jumped the previous year. .
Flumist reduced the risk from 57% to 62%.
For the H3N2 virus, the vaccine reduces by a little less than 40% the risk of serious infection of a child in those who received a vaccine the previous year and 23% who have not been vaccinated the last year.
The team found that Flumist was working better against H3N2. Children who had been vaccinated the previous year received a 50% protection against FluMist, but no protection against H3N2 if they had not been vaccinated before.
The team found that more often a child had been vaccinated before, better worked Flumist.
Researchers have found evidence that repeated vaccinations enhance immunity, especially in children. Vaccines stimulate both the production of antibodies and cells called T cells to fight germs. Repeated vaccination could more effectively enhance the action of T lymphocytes.
Maggie FoxMaggie Fox is the senior editor of NBC News and TODAY, where she writes major news on health policy, science, medical treatments and diseases.
She has been editor-in-chief of health technology and technology at the National Journal and senior editor of health and science for Reuters in Washington, DC and London.
Maggie has won awards from the Society of Business Editors and Writers, the National Immunization Program, the Overseas Press Club, and other organizations. She has made scholarships at Harvard Medical School, the National Institutes of Health, and the University of Maryland.
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