Lead author Jessica MacNeil of the Centers for Disease Control and Prevention in Atlanta and colleagues analyzed data for the current epidemiology and past trends in Haemophilus influenzae type b, or Hib, over the past two decades following the introduction of the Hib vaccine in the mid-1980s.
"The Hib vaccine was successful in reducing disease among children age 5 and younger, and now the epidemiology has changed," MacNeil said in a statement. "Most H. influenzae disease in the United States is now caused by other, non-type b strains of the bacteria."
However, the study authors warn the highest rates of disease from non-b type strains are in those age 65 and older and infants less than 1 year old. Many of these cases occur during the first month of life, and among those, premature and low birth weight babies are the most vulnerable, the researchers said.
Among adults age 65 and older who become ill due to H. influenzae nearly 25 percent of the cases are fatal, MacNeil said.
The findings were published in the journal Clinical Infectious Diseases.