Immunisation of infants against Haemophilus influenzae type b in the UK.

Haemophilus influenzae is one of the microorganisms that make up the commensal flora of the upper respiratory tract. Carriage is common, but compatible with good health. None the less, H influenzae is a pathogen and, in the case of encapsulated type b organisms, causes life threatening, invasive (bacteraemic) infections, the most common and serious being meningitis and epiglottitis (table 1).' The type b capsule is a crucial virulence factor and is composed of polyribosyl ribitol phosphate (PRP).2 Serum antibodies to the type b capsule can protect against invasive disease but immunocompetence generally takes some years to mature.3-5 Young children are therefore particularly susceptible to H influenzae type b and its potential to disseminate through the bloodstream (70% of cases occur in infants less than 2 years old).6 During the past two decades intensive efforts have been made to develop a vaccine that could hasten the onset of protective immunity with the goal of eliminating most serious H influenzae type b infections. There has been spectacular progress and this seems an opportune time to summarise the future of H influenzae type b immunisation in the UK.7 The crucial science that has been applied to developing effective vaccines againstH influenze type b for use in infants stems from observations made more than 60 years ago.8 Purified polysaccharides, such as PRP, are poor immunogens when used in their natural state and especially in children less than 2 years old.3 By linking the polysaccharide PRP chemically to a protein carrier, however, a conjugate