February 9, 2010

Standard Needle Size Not Always Adequate For Obese Teens

Researchers report in the Feb. 8 issue of the journal Pediatrics that using a standard 1-inch needle to immunize obese adolescents against hepatitis B virus produces a much weaker effect than using a longer needle.

Amy B. Middleman from Baylor College of Medicine in Houston, Texas wrote, "Obese youth achieve lower titers than average-weight peers in response to hepatitis B vaccine when a 1-inch needle is used."

The goal of the study was to evaluate whether using a longer (1.5-inch) vs. a standard (1-inch) needle to penetrate the thicker deltoid fat pad among obese youth who had not previously received the HBV vaccine would achieve higher antibody titers after immunization against HBV.

Over three years, Middleman and her colleagues 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.

Authors wrote, "This finding supports the hypothesis that needle length accounts for a significant portion of the discrepancy in immune response to HBV vaccine that is seen among those with obesity."

"As we continue to experience high rates of obesity in the United States and throughout the world, additional evidence-based research on optimizing the effective delivery of immunizations to adolescents and young adults will be critical," the study authors concluded. "Following updated needle length recommendations will be a first step toward improving the health of our youth and young adults by preventing vaccine-preventable diseases."

According to Reuters, Dr. Gregory Poland, who studies vaccines at the Mayo Clinic in Rochester, Minnesota, was not surprised with the results. For years, doctors have known that vaccines tend not to work as well in heavy people. It's unclear whether the obese have weaker immune systems or fat keeps shorter needles from reaching muscles.

With the obesity epidemic now adding extra insulation to the shoulder, "our needles are going to have to be longer," Poland told Reuters.

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."

Limitations of this study include sample size smaller than planned with loss of participants to follow-up, few male participants, limited ability to enroll patients who had not yet received HBV vaccine, and limited variability in race and ethnicity (most participants were Hispanic).

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."


On the Net: