Trends in Haemophilus influenzae type b infections in adults in England and Wales: surveillance study

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Abstract
Objective To describe invasive Haemophilus influenzae type b (Hib) infections in individuals aged 15 years or older in England and Wales between 1991 and 2003. Design Prospective, laboratory based surveillance of invasive Hib infections and cross sectional seroprevalence study. Setting England and Wales. Participants Cases were confirmed by isolation of H influenzae from a normally sterile site, or from a non-sterile site in cases with a diagnosis of epiglottitis. Excess serum samples collected from English 30-39 year olds as part of a national serosurvey were identified for the years 1990, 1994, 1997, 2000, and 2002. Main outcome measures The number of invasive Hib infections from 1991 to 2003. Population immunity to H influenzae type b in English adults was also measured. Results After routine infant immunisation was introduced in October 1992, adult Hib infections decreased initially but then rose from a low in 1998 to reach prevaccine levels in 2003. An associated fall in median Hib antibody concentrations occurred, from 1.29 µg/ml (95% confidence interval 0.90 to 1.64) in 1991 to 0.70 µg/ml (0.57 to 0.89) in 1994 (P = 0.006), with no significant change observed thereafter. Conclusions Although immunisation of infants resulted in an initial decline in Hib infections in adults, a resurgence in reported cases occurred in 2002-3. This rise was associated with an increase in cases in children and evidence of reduced immunity in older unimmunised cohorts. Childhood immunisation programmes may have unanticipated effects on the epidemiology of disease in older age groups, and surveillance strategies must be targeted at entire populations.