T helper cells and efficacy of Haemophilus influenzae type b conjugate vaccination.

A small number of fully vaccinated children in the UK have experienced invasive Haemophilus influenzae type b (Hib) infection. A rise in disease in recent years has been associated with lower vaccine-induced antibody levels over the first 5 years of life, forcing greater dependence on immunological memory for protection. This has necessitated the introduction of a catch-up campaign, designed to boost immunity in children aged 6 months to 4 years of age. We suggest that the conjugate vaccine's inability to induce pathogen specific helper T cells, combined with a loss of natural boosting due to reduced circulation of Hib, may have contributed to the rising incidence of invasive disease 10 years after introduction of the conjugate vaccine. If so, the changing epidemiology of Hib infection in the UK may in part reflect the failure of a subunit vaccine to activate adequately all the necessary components of the immune system. This observation has implications for optimal scheduling of more recently licensed meningococcal and pneumococcal conjugate vaccines.