Durham, NC—Pharmacists understand that, while the influenza vaccine is the best way to avoid becoming infected, it does not work equally in all patients.

What they might not know is that age is not the only determinant of how much protection is offered by immunization. A new study in the International Journal of Obesity points out that, even if they get a flu shot, obese adults remain twice as likely to contract the flu as adults of healthy weight.

To reach that conclusion, a study team involving researchers from Duke University and the University of North Carolina at Chapel Hill compared 1,022 adults from North Carolina who were healthy weight, overweight, or obese and who had received the seasonal trivalent inactivated influenza vaccine (IIV3) during the 2013–2014 and 2014–2015 flu seasons.

The study team then used laboratory tests and symptom scores to confirm whether a vaccinated participant had flu or influenza-like illness and to what degree their bodies produced influenza-fighting antibodies following vaccination.

Results showed that, although only a small percentage of the adults became ill with influenza after receiving the shot, the percentage was much higher in obese participants—9.8%—than those of healthy weight.

“Vaccinated obese adults are twice as likely to develop influenza and influenza-like illnesses compared to vaccinated healthy weight adults,” explained coauthor Melinda Beck, PhD, who posits that T cells of obese people might not function properly, thereby impairing responses to the vaccine.

Based on serum samples collected before and after participants received their vaccine, researchers found no differences in the antibody counts between vaccinated participants who did not report any symptoms and those who did, or between obese participants and people of healthy weight. The test was also unable to reliably predict whether a vaccinated obese person would have enough protection against influenza or not.

The samples were tested using the hemagglutination inhibition assay, a widely employed assay that indicates how many antibodies have built up in the body to fight off an infection such as influenza and also is used to evaluate candidate influenza vaccines.

Lead author Scot Neidich, PhD, of the Human Vaccine Institute at Duke University suggested such tests could, especially in the case of obese people, “provide misleading information.

“Impaired cell functioning, despite the robust production of antibodies, may make vaccinated obese adults more susceptible to influenza infection,” Neidich added. “Alternative approaches may be needed to protect obese adults from both seasonal and pandemic influenza virus infections.”

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