Surveillance of Haemophilus influenzae infection

Editor—Olowokure et al suggest that routine surveillance of Haemophilus influenzae is incomplete, that completeness declined after the vaccine was introduced, and that the effectiveness of vaccination programmes is overestimated.¹ We question this.

The authors suggest that the same weakness may affect the surveillance of the group C meningococcal vaccination programme. We accept that routine reporting is often incomplete, but, because of this, all vaccine preventable infections are under enhanced surveillance. The 15-fold reduction in the incidence of H influenzae type b disease after the introduction of the H influenzae type b vaccine was observed in an enhanced active surveillance scheme operating in five NHS regions,² not by the use of routine laboratory reports to the Public Health Laboratory Service (PHLS) Communicable Disease Surveillance Centre.

This surveillance scheme was established before the vaccine was introduced and was continued until 1995, when an enhanced national scheme was implemented.³ Completeness of data can be maximised by reconciling reports to the Communicable Disease Surveillance Centre, isolates referred to the PHLS Haemophilus Reference Unit, and notifications of H influenzae type b meningitis, and by active reporting of cases to the British Paediatric Surveillance Unit.

A similar scheme—reconciling reference laboratory reports, notified infections, cases known to consultants in communicable disease control, and laboratory reports to the Communicable Disease Surveillance Centre—was used to determine the burden of infection before the introduction of meningococcal group C vaccine⁴; this scheme has now been extended nationally.

Olowokure et al do not mention the most important weakness of surveillance systems for vaccine preventable disease. When the incidence of a disease is being ascertained after the introduction of a vaccine, the specificity of clinical case definitions and laboratory tests is critical. If specificity is low, when the true incidence of an infection declines the predictive value of the case definition falls and the proportion of false positive diagnoses increases.

Since 1990 in five regions, and since 1995 in the whole of England, all reports of confirmed invasive haemophilus infections have been followed up by referral of the isolate to the PHLS Haemophilus Reference Unit, where additional confirmation is carried out with molecular typing techniques. Between 1995 and 1999, of 136 putative type b isolates referred, only 108 were confirmed as type b.

The evaluation of surveillance data for H influenzae type b vaccine should be based only on confirmed type b infections. The collaboration between the Communicable Disease Surveillance Centre and the national reference laboratories for meningococcal and haemophilus infections ensures that national surveillance data for England and Wales for assessing the impact of vaccination are valid.