- Jodie McVernon, specialist registrar1,
- Caroline L Trotter, scientist (epidemiology)2,
- Mary P E Slack, consultant microbiologist3,
- Mary E Ramsay, consultant epidemiologist ()1
- 1 Immunisation Department, Health Protection Agency Communicable Disease Surveillance Centre, London NW9 5EQ
- 2 Statistics, Modelling and Economics Department, Health Protection Agency Communicable Disease Surveillance Centre
- 3 Health Protection Agency Specialist and Reference Microbiology Division, Haemophilus Reference Unit, John Radcliffe Hospital, Oxford OX3 9DU
- Correspondence to: M Ramsay
- Accepted 19 July 2004
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.
We thank all of the microbiologists and clinicians who have reported cases of invasive Hib over the past 10 years. Particular thanks are due to Sue Gurney of the Haemophilus Reference Unit for administrative assistance and Suzanna Stringer for carrying out the laboratory procedures. We also thank LouiseHesketh, Andrew Vyse, and Elizabeth Miller of the HPA seroepidemiology unit for providing the samples for this study. The antibody testing was carried out at HPA Porton Down and we thank Moya Burrage, Lorraine Ransley, Carol Powell, Janet Blake, Jenna Plank, and Annette Crowley-Luke from the immunoassay laboratory. We thank our colleagues at the National Reference Laboratory for Bacterial Meningitis, Netherlands, for allowing us to present their recent Hib incidence figures.
Contributors JM, MPES, and MER obtained and analysed disease incidence data. CLT obtained and analysed seroepidemiological data. All authors contributed to study design and preparation of the final manuscript. MER is guarantor.
Competing interests MER and MPES have received research grants from vaccine manufacturers. JM was previously employed in an academic research post that was funded by a vaccine manufacturer. MER, MPES, and JM have all received funds from vaccine manufacturers to attend conferences and meetings.
Funding The seroepidemiology component of this study was partially funded by the Public Health Laboratory Service Small Scientific Initiatives fund. Surveillance for invasive Hib infections is part of the core work of the Health Protection Agency's Communicable Disease Surveillance Centre and Haemophilus Reference Unit.
Ethical approval The Health Protection Agency has approval under Section 60 of the Health and Social Care Act to process confidential information about patients for the purposes of monitoring the efficacy and safety of vaccination programmes.