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Changing patterns of invasive Haemophilus influenzae disease in England and Wales after introduction of the Hib vaccination programme

^a Haemophilus Reference Unit, Oxford Public Health Laboratory, John Radcliffe Hospital, Headington, Oxford OX3 9DU, ^b Gwynedd Public Health Laboratory, Bangor, North Wales, ^c Public Health Laboratory Service, Communicable Disease Surveillance Centre, London NW9 5EQ

Correspondence to: Dr Hargreaves.

Since 1990 we have been monitoring strains of Haemophilus influenzae referred to the Public Health Laboratory Service Haemophilus Reference Laboratories from all cases of invasive H influenzae disease from five English regions and Wales. Methods of reporting and participating laboratories have remained constant over this period, which allowed us to compare the incidence of infection before and after the introduction of vaccination against H influenzae type b in October 1992.

Patients, methods, and results

The case definition was a systemic infection in which culture of normally sterile body fluid revealed H influenzae, or the organism was detected by antigen to H influenzae type b. Organisms were identified and typed at the reference laboratories using both type specific antisera and a polymerase chain reaction method.¹ Brief clinical details were also collected. The results for the first two years of the survey showed that most H influenzae infections were due to type b, presented as meningitis, and occurred in children under 5,² suggesting that mass vaccination of infants should achieve a rapid change in the pattern of invasive H influenzae infections.

Since October 1992 there has been a rapid reduction in the number of reported cases of H influenzae type b disease, particularly in children aged under 5 (see figure). Annual attack rates for H influenzae type b disease in children under 5 (calculated using denominator populations) have fallen from 30.9 cases per 100000 population in 1991-2 (369 cases recorded) to 2.0 per 100000 in 1993-4 (24 cases), a reduction in risk of invasive disease from 1 case in 3200 to 1 per 50000 children. Comparison of the rates of invasive H influenzae type b disease in children under 5 using log-linear regression showed a highly significant reduction (P<0.001 in 1993-4 compared with previous years).

Non-capsulate H influenzae isolates have shown an increase in annual attack rate (for all ages) from 0.25 cases per 100000 population in 1990-1 (45 cases recorded) to 0.37 in 1993-4 (67 cases). The total number of recorded cases of non-type b infections (non-capsulate and other serotypes: 75 cases) exceeded the number of cases of H influenzae type b (50 cases) in 1993-4. These increases demonstrate a sustained trend, approaching significance for non-capsulate infections during 1993-4 (P=0.066), which has been most noticeable in people aged over 65 years.

View larger version (111K): [in this window] [in a new window]   Invasive H influenzae type b and non-capsulate infections by quarter

Comment

These findings show the expected rapid reduction in the numbers of invasive H influenzae type b infections after the introduction of vaccination. The rate of decline has closely followed the increase in vaccine coverage and has been greatest in children aged under 5. This survey suggests that the United Kingdom vaccination schedule has been as effective at reducing numbers of cases as those schedules adopted in the USA^{3 4} and northern European countries.⁵ The increase in non-type b strains may reflect improved case ascertainment, perhaps because of increased awareness of haemophilus disease after the vaccination campaign. Nevertheless, there is a need to continue to monitor all invasive infections to determine whether these trends will be maintained in both the vaccinated and unvaccinated populations.

This survey was coordinated by the haemophilus working group of the Public Health Laboratory Service. We acknowledge the help of all contributing microbiologists, the regional coordinating microbiologists, and the staff at the haemophilus reference units in Oxford and Bangor. Dr Paddy Farrington gave statistical advice and Dr Mayon-White read the manuscript.

Funding: Haemophilus working group.

Conflict of interest: None.

1. Slack MPE, Crook DWM, Jordens JZ, Anderson EC, Falla T, Leaves NI, et al. Molecular and epidemiological aspects of Haemophilus influenzae infection. PHLS Microbiology Digest 1993;10:122-8.
2. Anderson EC, Begg NT, Crawshaw SC, Hargreaves RM, Howard AJ, Slack MPE. Epidemiology of invasive Haemophilus influenzae in the pre-vaccination era. Epidemiol Infect 1995;115:89-100. [Medline]
3. American Academy of Pediatrics. Haemophilus influenzae type b conjugate vaccines: Recommendations for immunization of infants and children 2 months of age and beyond. Paediatrics 1991;88:169-72. [Abstract/Free Full Text]
4. Adams WG, Deaver KA, Cochi SL, Plikaytis BD, Zell ER, Broome CV, et al. Decline of childhood. Haemophilus influenzae type b (Hib) disease in the Hib vaccine era. JAMA 1993;269:221-6. [Abstract/Free Full Text]
5. Peltola H, Kilpi T, Anttila M. Rapid disappearance of Haemophilus influenzae type b meningitis after routine childhood immunisation with conjugate vaccines. Lancet 1992;340:592-4. [Medline]

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