CHAPEL HILL — That annual flu shot may be significantly less effective if you're overweight, according to a new study by UNC-Chapel Hill researchers.
The vaccinations may be less likely to prevent flu if you're oversized and also less effective in reducing the illnesses' severity if you do catch it, according to the study, published today in the International Journal of Obesity.
"Basically what we're finding is that with increasing BMI (body mass index), from overweight to obese, the immune response to the vaccine is not as robust as it is for individuals who are at a healthy weight," said Melinda Beck, a professor and associate chairwoman of nutrition at the UNC Gillings School of Global Public Health and senior author of the study.
People who are overweight should continue to get the vaccinations, Beck said, because even a limited boost to the immune system may be enough to protect against flu. Also, the effects found in the study weren't universal to all the overweight participants.
The vaccine, usually reformulated each year to target the strains likely to be most prevalent, stimulates the body's immune system, which then generates antibodies that fight flu viruses.
The study involved 461 patients who were vaccinated in late 2009 at a UNC clinic and had blood samples drawn a month later, and a representative subset of 74 who were tested a year after the shots.
They divided the subjects into weight classifications based on body mass index, a simple way of estimating body fat based on height and weight.
The study found the level of those flu-fighting antibodies had jumped a month after the vaccination, reaching similar high levels regardless of body mass.
A second set of blood tests 12 months after the vaccination, though, showed antibody levels were significantly more likely to drop greatly among individuals who were overweight and obese.
The level fell four-fold among about half the obese subjects tested, but dropped that much in fewer than 25 percent of those whose weight was in the range considered healthy.
The researchers also studied a type of white blood cell that plays a part in the body's response to flu vaccine. These cells can reduce the severity of the illness if the antibodies produced by the vaccine don't prevent it.
A year after vaccination, those cells were significantly less likely to generate an infection-fighting protein in people who were overweight. This is the first study to show these effects of body mass on the immune response to flu vaccine, though obesity has been shown to reduce antibody response to vaccines for tetanus and hepatitis B.
The results of the study, Beck said, may offer an explanation for something researchers noticed during the 2009 H1N1 flu pandemic, that people who were overweight were more likely to contract and die from flu.
The findings also build on results of two earlier studies by the research team that showed overweight mice were less able to fight flu infections, more likely to die of them and appeared to have impaired immune responses to flu.
Researchers are still several steps away from looking into potential real-world changes that could help people who are overweight, such as a different form of vaccine or a second, "tune-up" vaccination part-way through the flu season.
First, Beck said, her team needs to determine whether the drops in antibodies and the ability of the white blood cells to fight flu really translate to a higher likelihood of catching the flu and getting a worse case.
It's possible even the weaker immune response among people who are overweight is enough to properly protect people from flu, Beck said.
"We still want people of all sizes to get vaccinated, because it may be enough," she said.