A new mouse study is raising concern that the flu vaccine may not protect people that are obese.

The findings come amid ongoing concerns about flu pandemics launched by avian flu viruses and the global rise of obesity. The World Health Organization estimates that 10 percent of adults worldwide and 42 million children under the age of 5 now qualify as obese. Obesity leaves individuals at increased risk for flu-related complications, including hospitalization and death.

In this study, researchers at St. Jude’s Research Hospital in Memphis, Tenn., used used vaccines prepared from killed viruses that are the basis of flu shots. The vaccines targeted an influenza A H1N1 seasonal flu strain as well as A(H7N9), a virus considered to have the potential to trigger a human pandemic.

Researchers looked at the immune response to vaccination in lean and obese mice, including how vaccine dose and the effect of different additives, called adjuvants, impacted that response. Both methods have been used to improve vaccine effectiveness in older adults and other high-risk groups.

While adjuvants improved the immune response to vaccinations in both lean and obese mice, the overall immune response was reduced in the obese animals compared to their lean counterparts. Following vaccination, the obese mice had lower antibody levels, including lower levels of neutralizing antibodies, and higher levels of the virus. In addition, lean mice receiving vaccines with adjuvants were protected from severe flu infections. Obese mice were not.

A four-fold increase in the dose of A(H7N9) vaccine bolstered the immune response in both lean and obese mice, but failed to protect the heavier animals from flu-related deaths. Protective antibodies from lean mice also failed to protect obese mice from flu infections.

These findings suggest that obese individuals might be at risk for flu infections even if their blood antibodies reach what have been considered protective levels, say the researchers of the study, which appears in the journal mBio.