BACKGROUND: Following the introduction of the Haemophilus influenzae serotype b (Hib) vaccine in 1985, Hib disease has decreased by >99% in the United States, though cases of invasive disease caused by other serotypes persist. Antibiotic prophylaxis is only indicated for Hib invasive disease and in Colorado, is dependent on obtaining serotype results from the state public health laboratory which can be delayed by long transport times and the need for re-culturing. Decisions about recommending antibiotic prophylaxis surrounding cases of invasive H. influenzae (Hi) can be difficult in the absence of serotype information or identifying clinical and epidemiological features of Hib in the post-vaccination era.
METHODS: We describe age and serotype distribution of reported cases of Hi in Colorado between 2007-2016 and discuss results from a survey of 13 Colorado public health agencies throughout the state about steps they take following an initial report of Hi prior to the availability of serotype information.
RESULTS: Of 709 cases of Hi reported from 2007-2016 in Colorado, 488 (68.8%) were non-typeable, 182 (25.7%) were serotypes a,c,d,e, or f and 25 (3.5%) were unknown. Only 14 (2.0%) cases of Hib were reported from 2007-2016; 6 of these (42.9%) were aged <5 years. Among cases <5, the predominant encapsulated serotype was a, which accounted for 17.9% of the total cases of Hi <5 years of age, or 26.3% of cases <5 when excluding neonates <30 days of age. Serotype b made up only 1.3% of cases ≥5 and 6.0% of cases <5. Of the 13 public health agencies surveyed, 6 (46%) said they initiated an investigation prior to obtaining serotype results while 4 (30%) said in the absence of serotype information, they consider factors such as age to determine how likely a case is to be Hib before deciding whether or not to start an investigation.
CONCLUSIONS: While Hi is prevalent, Hib is rare and most cases in Colorado are among adults. Other serotypes, in particular, Hia, are a more common cause of Hi among children <5 than serotype b. Public health agencies often consider factors such as age when making decisions about initiating an investigation in the absence of serotype but this is not supported by data. However, initiating an investigation surrounding all Hi cases in the rare event the case could be Hib may not be the best use of public health resources. Identification of Hib risk factors could assist in this decision making.