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If you’ve been diligent about getting your flu shot every year, you may not want to read this. But a growing body of evidence indicates that more may not always be better.

The evidence, which is confounding some researchers, suggests that getting flu shots repeatedly can gradually reduce the effectiveness of the vaccines under some circumstances.


That finding is worrying public health officials in the US, who have been urging everyone to get a flu shot each year — and who still believe an annual vaccination is better than skipping the vaccines altogether.

Dr. Edward Belongia is among the scientists who have seen the picture coming into focus. He and some colleagues at Wisconsin’s Marshfield Clinic Research Foundation reported recently that children who had been vaccinated annually over a number of years were more likely to contract the flu than kids who were only vaccinated in the season in which they were studied.

“The vaccine was significantly more effective … if they had not been vaccinated in the previous five years,” Belongia, an epidemiologist, recounted in a recent interview with STAT.


Vaccines work by exposing the immune system to a part of a disease agent — in the case of influenza, to two proteins on the exterior of the viruses — that has been rendered harmless. The vaccines tell the immune system to be ready to mount an offensive if it encounters the specified invaders.

The immune system then produces stores of protective ammunition — antibodies — it can use to fight off infection.

With many vaccines, an additional dose or two boosts the levels of antibodies in a person’s body. Some vaccines actually require multiple doses to be effective.

So the fact that repeated vaccination against flu might diminish rather than enhance the vaccine’s protection is perplexing.

It also represents a communications challenge for public health officials who vigorously promote annual vaccination as the most effective way to protect against the flu. Findings that suggest the science is more complicated than initially believed could lead people to assume annual flu shots are detrimental to their health.

That’s not the message researchers such as Belongia want to convey.

“In every scenario, it is better for people to be vaccinated than not vaccinated,” he said. “It would not be, I think, accurate or helpful for people to take away from this ‘Oh, well, I shouldn’t get vaccinated because I got vaccinated in the past and that’s a bad thing.’ ”

Like most issues related to mysterious and mercurial flu viruses, this one is a complex puzzle. But several researchers say the effect appears to be real — and needs to be explored further.

A number of countries are trying to get ever-larger portions of their populations immunized annually against influenza, a fact that makes it all the more important to figure out what is going on, flu experts say.

“The influenza immunization program is our largest, most costly annually repeated immunization program,” said Dr. Danuta Skowronski, an epidemiologist at the British Columbia Centre for Disease Control in Vancouver. “It’s worth it — so worth it — to invest in understanding these effects.”

But getting answers means mounting prospective, randomized clinical trials, and that will be both expensive and complicated.

The work cannot be done in the United States, where the Centers for Disease Control and Prevention has recommended since 2010 that everyone receive annual flu vaccinations. Given that policy, it would be unethical for researchers here to randomly assign some people to forgo vaccinations in some years. But experts elsewhere, including in Hong Kong, where influenza circulates year-round, are trying to put together the funding for what would have to be a large, multiyear study.

The question of the effectiveness of repeated flu vaccines has actually been kicking around for decades. Back in the 1970s a researcher noticed that children at a boarding school who were vaccinated year after year seemed more likely to catch the flu. Later studies disputed the suggestion.

Like most issues related to mysterious and mercurial flu viruses, this one is a complex puzzle.

Then in 1999, a leading influenza researcher, Derek Smith, suggested that in years when a component of the vaccine — say the part that protects against the influenza A family called H3N2 — had changed little or not at all from the previous year’s vaccine, the second year’s vaccine would induce less protection. Smith, now based at Britain’s University of Cambridge, called it negative interference.

The idea is that the antibodies produced in year one may neutralize some of the vaccine in year two’s shot before it can trigger a full immune response, explained Dr. John Treanor, a vaccine expert at the University of Rochester Medical Center in New York.

Smith also argued that when the vaccine viruses were quite different from one year to the next the recipient would actually get enhanced protection. Positive interference, he called it.

Skowronski started to see evidence of negative interference in the mid-2000s, when she and a fellow Canadian researcher, Dr. Gaston De Serres of Quebec’s public health agency, found the flu vaccine was significantly less effective than they had expected. Conventional wisdom at the time was that it cut the risk of catching the flu by 70 percent to 90 percent. But even during flu seasons when the vaccine was well-matched to the viruses making people sick, it wasn’t proving that effective.

In their search for answers, the researchers considered the people they were studying. About 90 percent were getting flu shots every year. “They’re habitual immunizers,” Skowronski said.

Figuring out whether negative interference is real and what can be done about it is important, Treanor said. But if the phenomenon really exists, researchers have unearthed a problem without an immediate solution.

That’s because influenza vaccine protects against three or four different families of flu viruses. The vaccines only come in the combination form.

A number of research teams are working to develop a universal flu vaccine, one that trains the body’s immune system to fight off all flu viruses. The goal is to have a vaccine people might need to take only a few times in their lives, maybe once a decade. That might solve the problem, Treanor said — but he noted a universal flu vaccine could be years away.

In the meantime, high-dose vaccines might help ensure that vaccines don’t become less effective over time. The extra vaccine in the shot might override the dampening effect of previous years’ antibodies. But Treanor cautioned that theory hasn’t yet been put to the test. And the sole high-dose flu vaccine available in the US — made by Sanofi Pasteur — is licensed for use only in adults 65 and older.

Likewise, an adjuvanted vaccine — one that includes a compound that boosts the immune response the vaccine generates — might prove effective. Although none is currently licensed in the United States, one may be coming.

As for changing the frequency with which the flu vaccine is given, it’s far too soon to even contemplate that kind of move, Belongia said.

“The policy of vaccinating every year has been generally successful,” Belongia said. “We wouldn’t want to change that unless we know for sure that we’re changing it to something that’s going to be better. And right now I don’t think we have any good idea what that would be.’’

  • I’m 48 and have had one, maybe two flu shots in my teens. I have had the flu a few (less than five?) times my whole life and certainly not in the last 15 years. This not say that I am anti-vax. Polio and Measles are a whole different thing. I do try to avoid antibiotics whenever possible. My two cents

  • “The work cannot be done in the United States, where the Centers for Disease Control and Prevention has recommended since 2010 that everyone receive annual flu vaccinations. Given that policy, it would be unethical for researchers here to randomly assign some people to forgo vaccinations in some years.”

    Why would it be “unethical” to ignore such a policy if indeed it is meritless? Would it not instead be unethical to accept the enforcement of such a policy ?

  • I don’t see how it is unethical when so many people willingly forego flu vaccination in the first place, myself included. Do the study. While you’re at it, do a legitimate study comparing vaccinated and non-vaccinated individuals and see if there are any differences among them when it comes to immune function and chronic illnesses (including autoimmune conditions).

  • Last year, after vaccinating for about 10 years in a row, I caught 6 infections, 2 of them flu. This year I did not vaccinate, but have been taking 4000 units of vitamin D instead. So far I only caught one mild and short lived infection.

    • I agree that our bodies need help to fight off infection and viruses , the flu jab is one way ..but today our bodies are under stress and this itself can and does strip our bodies of natural protection , I take 1000mg of Vitamin C, plus Magnesium, and Vit D ..with the flu jab , seems to help a great deal as at one time I was always getting chest infections ..and I’ve now added Tumeric to the list ..and helps me , greatly combo ..grief and divorce are classic ways that strip your bodies of any protection ..especially for woman after the menopause , again ,their bodies lose needed protection and fail in most cases to address this ..sadly , thinking it’s too late ..not so ..

    • Actually a link went to the primary source about interference but it didn’t conclude what is suggested here but to the contrary concluded Hoskins paradox is unsubstantiated by his own data.

  • Oh the smart scientists are befuddled are they!? well, this mother isn’t… There is a very simple and logical, and even expected explanation for this phenomena. One that every person who has studied vaccine efficacy and safety (yes, peer reviewed/published) for little more than 60 minutes knows well. That is that when one receives an influenza vaccine, the increased immunoresponse is very short term, providing increased protection for only several weeks or months (often not even long enough to make it through an entire flue season.) Those who are vaccinated, in essence avoid illness in the short term, but are no longer protected at all next season. One who has not received influenza vaccinations often or at all, has (hopefully) contracted influenza several times, and had a an immune response each time that helped them recover. The immunoresponse to the actual disease is much longer lasting and provides ongoing protection for many years, perhaps some life long. If that person then participates in a study and gets a flu vaccine once, they will have better influenza immunity going into the study than anyone who receives the vaccine annually. It looks like the vaccine works better, but really it is the test subject’s own immune system that changes the curve.
    I hope that helps out the near genius medical doctors and researchers who paid hundreds and thousands for their education. But I doubt the theory I proposed will be studied…if correct, it could be awful for vaccine sales and beneficial for people’s health, and who could stand for that?

    • Not to be mean, but an uneducated mom is NOT someone I would trust over educated scientists. Sorry Nancy. Your untested critical thinking skills are not going to guide my decision making.

    • @Bob … not to be mean, but what bigotry exists in your mind that “mother” and “uneducated” are intrinsically, mutually exclusive? You drink whatever Kool aid you want Bob. Just allow me the freedom not to partake of it.

    • Nancy i like your theory. I’m one of those people who haven’t taken the shot. I have, in the last 2 decades, been sick 1-2 times, but i’ve not had the flu for about 10 years. People ask me “did you get your shot?” and just to pacify them i lie and say “yeah, did you!?” because you never know if it’s one of those zealous preachy and pushy ones.

  • Can’t we do a simple study that shows people who choose not to have the immunization and the level of the illness they may contract versus those who get sick regardless of the fact that they received an immunization and those who remain healthy.
    This is simple tracking and shouldn’t be complicated. Having a very expensive nation wide immunization routine without tracking effectiveness is like someone blindfolded aiming and shooting a golden arrow at an ? apple and never removing the blindfold to test their accuracy.
    Another question- anyone keeping track of the 2 to 3 days of malaise often suffered by those who do receive the immunization?

    • They have. Cochrane collaboration metadata study of flu vaccine vs non vaccine. Flu vaccine offers little protection from the flu and increases RSV and other illnesses. You are sicker for longer with the flu vaccine with the flu or with other illness. Just. Say. No. This yearly shot is setting us up FOR a pandemic.

  • It baffles me that people continue to think flu shots are good for them. It’s maddening that advertisements sway the public into getting them. There is so much information about vaccines and the damage they can cause. It takes time, but you have to inform yourself.

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