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Your weight could make the flu shot less effective, so that’s fun

The effectiveness of the flu vaccine may be compromised by your weight, according to a new study published in mBio. Researchers found that being overweight seemed to decrease the effectiveness of the shot, and being obese may make it fail altogether — a significant problem, as obesity already heightens your risk of complications from the flu.

To investigate this connection, scientists tested two groups of mice — one lean and one obese. Each group received the flu shot and were then exposed to the influenza virus. They monitored the animals to see which ones fell ill, and took blood samples to measure the level of their immune response. Not only did the obese mice have a lower level of flu-fighting antibodies, but the vaccine seemed to provide them no protection at all. And while the study was done on rodents, the researchers say it has serious implications for human beings too.

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One of the most common factors that affect how well a medication works is your weight, yet we hear so little about this well-known fact among doctors. One reason may be the loaded nature of the topic. “Unfortunately the reality is, most doctors don’t discuss this with their patients,” says Robert Grossman, M.D., of Los Robles Hospital in California. “It’s a very uncomfortable conversation to have.”

Another reason may be that it’s a woefully understudied topic. “FDA studies generally exclude the obese, by practice. They choose populations with no existing issues to control variables and get ‘cleaner’ data,” Grossman says. “So often this type of information doesn’t come out until ‘after-market’ studies, at which point you have to really look at the data to understand which patients are susceptible to which side effects.”

The effectiveness of the flu shot in general is a matter of constant debate. Because scientists have to predict which flu strains will be most serious up to a year in advance, the shot may not provide great protection even under ideal circumstances. Last year, for instance, the shot was just 23 percent effective. Even when it’s considered a good match, like the one forecast for the 2016-2017 flu season, it’s still just 59 percent effective. So anything that could worsen that number, like the patient’s weight, is vital information.

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To be clear, this should not be a reason to fat-shame people. People of any weight and size deserve the best medical protections they can get. Rather, this seems to be a failure on the part of the medical community to create a dose or formulation for the flu shot that would be effective for the largest number of people. There will never be a perfect flu shot, but it seems like the least they could do would be to offer dosages calculated on weight instead of one set “adult” dose. There is currently a “high-dose” flu vaccine that is used primarily in seniors. It is unknown whether this dosage would be more effective in overweight or obese patients, but it may be worth asking your doctor about.

In the meantime, it’s always a good idea to take what measures you can to protect yourself and others during cold and flu season, like always washing your hands, eating a diet rich in fruits and vegetables and staying home if you get sick.

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