NEW YORK (Reuters Health) - The incidence of invasive Haemophilus influenzae disease appears to be increasing among adults, especially in those 65 years and older, according to a report in the March 15th Clinical Infectious Diseases.

Furthermore, invasive H. influenzae disease in adults "is mainly not the strain covered in the childhood vaccine," Dr. Mark S. Dworkin told Reuters Health. "If the disease continues to increase in this population, it raises the question whether a vaccine will need to be developed to try to protect the elderly."

Dr. Dworkin, from the University of Illinois at Chicago School of Public Health, and colleagues examined the epidemiological changes that have occurred since the introduction of H. influenzae type b (Hib) vaccine for children in the United States.

Between 1996 and 2004, the number of cases of invasive H. influenzae disease reported to the Illinois Department of Public Health increased by 169%, the team reports.

Compared with 1996, cases of invasive H. influenzae in 2004 were 2.9 times more likely among persons aged 18-64 years and 3.6 times more likely among those aged 65 years and older, the results indicate.

Most of the isolates tested were nontypeable (54.2%) or encapsulated non-b (30.8%), the researchers note, and only 14.9% were type b.

Only the number of type b cases decreased during this interval, the report indicates, from 15 cases per year during 1996 to no more than 6 cases per year between 2001 and 2004.

"A study like mine is an example how reporting disease information can be helpful," Dr. Dworkin said. "It allows for trends over time to be studied to see the impact of a vaccine (like Hib declining markedly in the 1990's) and of the possible emergence of non-vaccine serotypes (as is suggested in our data)."

The authors of a related commentary, Dr. Anne Schuchat and Dr. Nancy Rosenstein Messonnier from the Centers for Disease Control and Prevention in Atlanta, Georgia, write: "The intriguing surveillance analysis of Dworkin et al. ... highlights the value of long-term monitoring of immunization programs through laboratory-based surveillance efforts that can differentiate vaccine-preventable disease from other cases."

"This is especially important as new vaccines, including combination vaccines, become available," the editorialists conclude.