Transmission dynamic models of meningococcal carriage, disease and the impact of serogroup C conjugate vaccination

The meningococcal serogroup C conjugate (MCC) vaccine was introduced into the UK immunisation schedule in 1999. MCC vaccine was incorporated into the routine vaccine schedule at 2, 3 and 4 months of age, and at the same time, a catch-up campaign targeting all children under the age of 18 years was launched. The MCC vaccine has successfully controlled serogroup C disease in England and Wales, due to high short-term vaccine effectiveness and substantial herd immunity, resulting from a reduction in serogroup C carriage. A realistic age-structured transmission dynamic model was developed and applied to predict the direct and indirect effects MCC vaccination. The model was parameterised and fitted to epidemiological data from England and Wales and was used to investigate the effects of a range of vaccine strategies and scenarios. The number of serogroup C cases predicted by the model in the five years following vaccine introduction fitted well to the observed number of cases in England and Wales. Comparison of vaccine strategies showed that immunising teenagers, the age group who are the most common carriers of meningococci, was most effective in generating herd immunity. Vaccine effectiveness studies have shown that protection wanes rapidly in infants immunised at 2, 3 and 4 months of age. The model predicts that few cases of serogroup C disease will occur in the next few years, even in the absence of changes to the vaccine schedule, because the herd immunity generated from the catch-up campaign will persist for several more years. More cases were predicted to occur if we assumed the duration of protection against carriage was shorter than protection against disease. The continued success of the MCC vaccine program, despite the failure of the vaccine to provide long term protection to children immunised in infancy, must be attributed largely to the reduction in transmission following the catch-up campaign. This has important implications for the design and evaluation of other meningococcal disease control programs, including those against serogroup A disease in the meningitis belt in sub-Saharan Africa.