Two versions of the Hib vaccine, which protects against serious infections caused by a bacteria called Haemophilus influenzae b, appear to have different effects in elderly adults and infants, new research shows.

The one that is most effective in elderly adults is least effective in infants and vice versa.

This finding suggests there are some “fundamental differences” between these age groups in their immune response to Hib vaccines.

In the study, Dr. Kathleen R. Lottenbach, from St. Louis University, and colleagues found that a Hib vaccine containing an immune-boosting compound called diphtheria toxoid (D) was highly effective in older adults, while one containing a different booster called Neisseria meningitidis protein (OMP) didn’t work as well. As noted, the reverse situation is usually seen in infants.

The findings, which appear in the Journal of the American Geriatrics Society, stem from a study of 125 older adults who were injected with D, OMP, or plain Hib vaccine.

The authors found that the OMP vaccine induced protective antibodies in the study group, but the response was just not as robust as that seen with the D vaccine.

The antibody response observed with the D vaccine was higher than that achieved with the plain vaccine. Moreover, at 1 year, subjects treated with the D vaccine had higher antibody levels than those who received the OMP vaccine.

“The results of this study provide further insights into factors affecting…antibody responses to Hib vaccination of the elderly,” the authors state.

SOURCE: Journal of the American Geriatrics Society, November 2004.