Non-Hib infections cause substantial disease in both elderly and young
MedWire News: Study findings show that vaccinating children against Haemophilus influenzae serotype b (Hib) has dramatically reduced disease incidence over the past 20 years, although other strains of the bacteria continue to cause substantial disease in the oldest and youngest age groups.
"The Hib vaccine was successful in reducing disease among children aged 5 years and younger, and now the epidemiology has changed," said lead study author Jessica MacNeil (Centers for Disease Control and Prevention, Atlanta, Georgia, USA) in a press statement.
MacNeil and team analyzed active surveillance data for current epidemiology and past trends for invasive H. influenza disease conducted through Active Bacterial Core (ABC) surveillance sites during 1989-2008. Incidence rates were calculated using US census data for the ABC sites, and estimates of the number of cases and deaths were calculated standardizing for race and age.
During 1999-2008, 4838 cases of H. influenzae disease were reported, resulting in an estimated mean annual incidence of 1.62 cases per 100,000 population. Of these, 15.3% were fatal. Incidence was found to be highest among children aged younger than 1 year and in adults aged 65 years or older compared with those in other age ranges (9.54 and 6.31 vs 0.35-1.70 cases per 100,000 population, respectively).
The estimated national incidence of H. influenzae disease for children aged 2-4 years, 1 year, and less than 1 year was 1.14, 2.03, and 9.54 cases per 100,000 population, respectively. Furthermore, many of the cases occurring in infants younger than 1 year were seen during the first month of life in preterm or low-birthweight infants.
The researchers also found that an estimated 10% of the total burden of disease among children aged younger than 5 years occurred in American-Indian and Alaskan-Native Children.
"Why these groups continue to be at higher risk than other populations should be the focus of future studies," said MacNeil.
No significant serotype replacement occurred among children, suggesting that the current Hib vaccine has been effective in preventing H. influenzae disease in this age group.
Analysis of trends in disease incidence from 1989 to 2008 showed that in 1989, 79.7% of invasive disease was caused by Hib and 16.8% by nontypeable strains. However, by 2008, only 3.0% of invasive disease was caused by Hib and 68.4% by nontypeable strains.
Despite a 65% decrease in mean annual incidence of H. influenzae infection, incidence remained stable (12% decrease) among adults aged 65 years or older.
"Studies are needed to identify risk factors for disease to determine how to maximize benefits from interventions designed to decrease incidence and severity in these age groups," write MacNeil and team in Clinical Infectious Diseases.
By Ingrid Grasmo