NEW YORK (Reuters Health) - Our ever-expanding waistlines may have outgrown the doctor’s needle, researchers say, in what could be another casualty of the obesity epidemic.

A medical assistant injects a vaccine in a file photo. REUTERS/Michael Buholzer

In a new study, the researchers report that using a standard 1-inch needle to immunize obese adolescents against hepatitis B virus produced a much weaker effect than using a longer needle.

“As obesity rises in the US, we need to be aware that the standard of care may have to change to protect obese youth,” study co-author Dr. Amy Middleman of Baylor College of Medicine in Houston told Reuters Health.

Over three years her team vaccinated 22 young women and two young men in the shoulder, randomly assigning them to be injected with either a 1-inch or a 1.5-inch needle.

Once injected, vaccines trigger production of small molecules called antibodies, which kick-start our immune system if we are ever attacked by the virus again.

The two groups turned out to have different antibody counts depending on the needle used. In those injected with the short one, the number was almost halved.

Although everyone in the study had enough antibodies to be considered protected against hepatitis B, a lower count generally means a less robust response.

“It gives us more evidence of the importance of choosing the right needle length,” said Middleman, “because we just don’t know what the impact could be in other vaccines.”

The results are no surprise, said Dr. Gregory Poland, who studies vaccines at the Mayo Clinic in Rochester, Minnesota. For years, doctors have known that vaccines tend not to work as well in heavy people. Whether the obese have weaker immune systems, or fat keeps shorter needles from reaching muscles, where the vaccines can affect immune cells, was unclear.

The introduction of the hepatitis B vaccine in the 1980s offered some clues. Soon after doctors began using the vaccine, they realized that it was failing to protect some female nurses.

At the time, the shot was given in the buttock, Poland told Reuters Health, and was thwarted by the padding there.

Instead of entering the muscle as it was supposed to, the vaccine apparently was broken down in the fat tissue, where it had little chance to affect immune cells. So doctors began giving the shot in the less-padded shoulder.

With the obesity epidemic now adding extra insulation to the shoulder, “our needles are going to have to be longer,” Poland said.

And those long needles aren’t as unpleasant as they sound. In fact, Poland said, “they turn out to be less painful and have fewer side effects.”

The Centers for Disease Control and Prevention recommends longer needles in obese patients, but it is unclear how many doctors follow these guidelines, or even know about them.

Women are more affected by needle length than men, because their fat distribution is different. But even obese women shouldn’t be overly concerned until more research has been done, Middleman cautioned.

“Should you go back and get all your immunizations repeated?” she said, “No, I don’t think so.”

SOURCE: Pediatrics, March 2010 (published online February 8, 2010).