The flu shot may be less effective in people who are obese, according to a study published Tuesday in the International Journal of Obesity.

The study found that people vaccinated against the flu who were obese were twice as likely to develop influenza or influenza-like illnesses than vaccinated people of healthy weight.

This finding is troubling for a couple of reasons. First, the effectiveness of the flu vaccine is already somewhat low — about 50 percent to 60 percent among the overall U.S. population during years when the viruses in the vaccine are similar to those in circulation, according to federal health officials.

Second, we are in the midst of an obesity epidemic. Nationally, one in three adults are obese. Here in Minnesota, the rate is lower, but still too high — about one in four.

Also, since the H1N1 flu pandemic of 2009, obesity has been recognized — along with age, pregnancy and certain chronic diseases — as an independent risk factor for developing complications from influenza, including dying from the illness.

Study details

For the new study, researchers at the University of North Carolina at Chapel Hill analyzed data collected from 1,022 North Carolinian adults who had received the influenza vaccine during the 2013-2014 or 2015-2016 flu seasons. The participants were categorized as healthy weight, overweight or obese based on their body mass index (BMI). Overall, 27 percent of the participants were healthy weight, 28 percent were overweight and 44 percent were obese.

Each week during the flu season, the participants were contacted by phone or e-mail and asked to report any symptoms of fever, cough, runny nose, sore throat, muscle aches, headaches and fatigue. They were also instructed to call a nurse associated with the study if they developed such symptoms. At the end of the study, the participants’ medical records were reviewed to determine whether they had developed a confirmed case of influenza (based on blood tests) or an influenza-like illness (based on the reported symptoms).

Seventy-four of the study’s participants did become ill with either a laboratory-confirmed case of influenza (10) or an influenza-like illness (64). But the risk was higher among those who were obese: 9.8 percent of them became ill versus 5.1 percent of the normal-weight participants.

A weaker immune response

The study’s authors believe the reason the obese people got the flu is that their T cells, which play a central role in developing the body’s immune response to influenza viruses (and other pathogens), are not functioning properly.

Other research has shown that T cells are similarly impaired in vaccinated elderly people.

The study found that blood samples taken before and after the flu shot was administered did not reveal any difference in antibody counts between people who got the flu and those who didn’t, or between people who were obese and those who were not. Nor did the samples reliably predict whether an obese person had enough antibodies to protect against influenza.

These findings suggest, say the researchers, that the current standard blood tests used to indicate whether a person has enough antibodies to protect against influenza are unreliable.

“Impaired cell functioning, despite the robust production of antibodies, may make vaccinated obese adults more susceptible to influenza infection,” said Scott Neidich, the study’s lead author and a doctoral student at the University of North Carolina, in a released statement. 

“Alternative approaches may be needed to protect obese adults from both seasonal and pandemic influenza virus infections,” he added.

FMI: You’ll find an abstract of the study on the website of the International Journal of Obesity, but the full study is behind a paywall.