You followed your doctor's advice and got a flu shot, and now you're protected from getting the flu this year. Right? Not so fast. If you're overweight, you may need extra protection. A study from the University of North Carolina at Chapel Hill found that obesity may make annual flu shots less effective.
The research, published in the International Journal of Obesity, backed up the observation during the 2009 H1N1 outbreak that obese people seemed to have a weakened immune response to influenza vaccinations.
"These results suggest that overweight and obese people would be more likely than healthy weight people to experience flu illness following exposure to the flu virus," said Melinda Beck, Ph.D., professor and associate chair of nutrition at the UNC Gillings School of Global Public Health and senior author of the study.
"Previous studies have indicated the possibility that obesity might impair the human body's ability to fight flu viruses," she said. "These new findings seem to give us a reason why obese people were more susceptible to influenza illness during the H1N1 pandemic compared to healthy weight people."
The study found that influenza antibody levels declined much more rapidly in obese people when compared to individuals of normal weight. In addition, levels of CD8+T cells (a type of white blood cell that's key in the body's immune system) are defective in overweight people.
After the initial vaccination in 2009, both overweight and normal weight individuals all developed antibodies to the flu virus within a month. But antibody levels fell much more quickly in overweight and obese people over the course of a year. Antibody levels dropped fourfold in about 50 percent of obese individuals, while less than 25 percent of normal weight individuals had a fourfold decrease.
In addition, when study participants' blood samples were exposed to a flu virus 12 months after vaccination, only about 25 percent of obese patients' cells still expressed the flu-fighting protein compared to about 75 percent of individuals of normal weight.
Heather Paich, a doctoral student in Beck's lab, added: "The findings also suggest overweight and obese people are more likely to become sicker and have more complications."
Earlier research by Beck had found that the fatality rate among obese mice with influenza was significantly higher — none of the slim mice died while 25 percent of the obese mice died.
"We need to continue to study the effect of obesity on the ability to fight virus infections," Beck said. "Influenza is a serious public health threat, killing up to half a million people a year worldwide. As rates of obesity continue to rise, the number of deaths from the flu could rise, too. We need to better understand this problem and to look for solutions."
According to the CDC, an average of 5 to 20 percent of the population gets flu each year, and more than 200,000 are hospitalized due to complications. During the past 30 years, deaths associated with flu run from a low of 3,000 a year to a high of about 49,000 people, depending on the severity of the flu season.
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