For obese people, vaccine needle size matters

NEW YORK — Our expanding waistlines may have outgrown doctors’ needles, researchers report in the March issue of Pediatrics.

In the study, they write 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 U.S., 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 women and two 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 antibodies, which kick-start our immune system if the virus ever attacks.

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," Middleman said, "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, Minn. 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.