The Centers for Disease Control and Prevention says flu season is starting to ramp up — and it's not too late to reduce your risk with a vaccine.

But scientists have come to realize that flu vaccines are less effective for people who are overweight or obese. Considering that excess weight affects more than two-thirds of the U.S. adult population, that's a significant shortcoming.

Researchers are studying why that's the case, with an eye toward developing better flu vaccines.

This issue came to light during the 2009 flu pandemic, the first major outbreak of the 21st century. Health officials noticed that the flu was taking a particular toll on people who were significantly overweight.

"We had never seen that before," says Stacey Shultz-Cherry, an infectious disease specialist at St. Jude Children's Research Hospital.

"The virus is able to grow to higher [concentrations] and spread deeper in your lungs, which is not what you want during an influenza infection," she says.

And people weren't simply getting sicker — they were also more likely to spread the disease. That has the potential to amplify a flu outbreak.

Shultz-Cherry says this point is highlighted in a study of volunteers at the University of Maryland.

"The students who were overweight/obese actually had more virus coming out in their exhaled breath," Shultz-Cherry says.

Another research team studied families in Nicaragua during flu season and found that overweight people shed the virus by an extra day, on average, compared with people of lower body weight. That effect was bigger for people who had no or few symptoms.

"What's particularly disturbing for us is the fact that the vaccine doesn't work as well" in this population, says Melinda Beck, a professor of nutrition at the University of North Carolina at Chapel Hill. She's been studying the effect of obesity on a person's ability to fend off infections, "and influenza is one of my favorite viruses, so that seemed like a natural thing to study."

She looked at the two different parts of the immune system: one involves antibodies, which are proteins that can neutralize viruses; the other involves cells that join in on the fight against disease.

"You usually think of the vaccine as protecting you with antibodies," she says. "And in fact, obese people could make a nice antibody response." The part of the immune system that fails in this case involves the T cells, she says.

As people become overweight, their metabolism changes. And Beck says that metabolic shift affects many cells, including immune system cells. She has studied the details of this in mice.

Elderly people face the same problem. "Sometimes they can make an antibody response, but it's their T cells that aren't functioning, so they can still get infected with flu even though they've been vaccinated," she says.

As a result, a "30-year-old obese person has the immune cells that look a lot like what you might expect in an 80-year-old individual," Beck says.

That may be why flu vaccines don't work so well in older people, either. So it's possible that improvements in the vaccine will help both people who are older as well as those who are overweight.

Shultz-Cherry is part of a new effort at the National Institutes of Health to come up with a next-generation flu vaccine.

"That's exactly our hope — that we can find a vaccine that will work for everybody, but especially for these higher-risk populations," she says.

That effort will take many years. In the meantime, Shultz-Cherry says it's still important for everyone to get vaccinated. People of all body types are at higher risk of heart attack or stroke if they get the flu.

There's a lot of reasons to get the flu shot," she says, "even if it doesn't work as well as we want in this high-risk population."

You can reach NPR Science Correspondent Richard Harris at rharris@npr.org.