A study published in Pediatrics suggests that, when compared with standard length needles (1.0-inch), a longer (1.5-inch) needle yielded better results in obese adolescents after immunization with hepatitis B vaccine (HBV). Although all the participants in the study had enough antibodies to be considered protected against hepatitis B, the response was markedly less robust when the shorter needle was used.
The goal of this randomized trial was to evaluate whether using the larger 1.5-inch needle to vaccinate obese adolescents and young adults who had not previously received the HBV vaccine would achieve higher antibody titers after immunization. The Texas-based investigators randomly assigned 65 obese young people from the Houston area to be immunized with HBV vaccine using either a 1-inch or a 1.5-inch needle. Median age was 21, with a mean body mass index of 40. Those younger than 19 received a 0.5-mL injection; older participants received a 1.0-mL dose. The participants received three doses of vaccines at baseline, 1, and 4 months, and blood samples were taken at baseline and again at 6 months. Most of the participants were Hispanic, and all but one participant included in the final analysis was female.
Of the 65 participants, 17 were disqualified because they had evidence of prior HBV immunization. Twenty-four participants moved, withdrew from the study, or did not respond to follow-up communication. At the end of the study, 10 participants had completed the 1-inch protocol, and 14 had completed the 1.5-inch protocol. The participants vaccinated with the 1.5-inch needle had a mean antibody titer that was 54% higher than the participants in the 1.0-inch group (345.4 mIU/mL versus 189.8 mIU/mL; P=0.03). The major limitations to the study are the small sample size, the disproportionately high number of females, a limited ability to enroll participants who had not yet received HBV vaccine, and the lack of patient and family investment in the project.
This study, although limited by design and implementation problems, corroborates other studies suggesting that obese young people have a blunted immune response following HPV vaccination. This is likely due to the inability of a standard-length needle to fully penetrate adipose tissue and be absorbed into the muscle. Adipose tissue has a less abundant blood supply that may delay the immune response to B and T cells. Although these data provide information regarding the immune response in obese adolescents, the study provides no documentation as to whether the difference in antibody response translates to any clinical significance. While the authors emphasize the need for a review of injection practices in obese participants, they fail to address the overriding problem, which is the growing number of obese young people in the United States.