[Increase in the number of invasive Haemophilus influenzae type b infections].

OBJECTIVE: To describe the increase of invasive Haemophilus influenzae type b (Hib) infections in The Netherlands before and after the introduction of Hib vaccination in 1993, and to hypothesise about possible explanations.
DESIGN: Descriptive.
METHOD: Data on the prevalence of invasive Hib infections, such as meningitis and epiglottitis, during 1990-2004 were obtained from The Netherlands Reference Laboratory for Bacterial Meningitis, which collects Hib isolates from spinal fluid and blood from across the country.
RESULTS: The incidence of invasive Hib infections decreased substantially for a few years after 1993. The total number of isolates was at a minimum in 1999 (n = 12) and increased to 49 in 2004. The annual number of patients with vaccine failure was 5 or less during 1995-2001, but was between 10 and 15 from 2002 onwards. A definite explanation for the increase in the incidence of invasive Hib infections cannot be given. Improbable causes are a surveillance artefact, an impaired response to the vaccine due to vaccination-scheme changes or interaction with other vaccines, or selection of Hib variants that are less sensitive to the vaccine-induced immunity. It most likely involves secondary vaccine failure: Hib carriership is decreased by mass vaccination, whereupon natural boosting occurs less frequently later in life. Subsequently, immunity decreases and susceptibility to invasive infection increases. Careful surveillance of invasive Hib infections in The Netherlands remains important.