In November 1999, the U.K. introduced meningococcal serogroup C conjugate (MCC) vaccines for children less than 18 years of age. A total of 12 million children in England and Wales were eligible for immunisation and age groups were prioritised for receipt of the vaccine beginning with the 15 to 17 year olds in Nov 1999 and ending with 5 to 8 year olds by the end of Oct 2000. Enhanced surveillance was introduced to accurately monitor disease incidence and identify possible cases of vaccine failure. Vaccination history was obtained for all serogroup C meningococcal cases confirmed in targeted age groups. Efficacies estimated for up to 21 months of follow up using the screening method. Evidence of herd immunity was determined by comparing incidence of serogroup C disease in unvaccinated individuals before and after the MCC campaign.

By end of December 2001 overall coverage of approximately 82% in children aged 5 years and over and 90% in those less than 5 years of age had been achieved. Large reductions in serogroup C disease have been observed in all vaccinated age groups. Up to the end of December 2002, 41 confirmed vaccine failures had been identified. Efficacy estimates for up to the end of December 2002 in England are; for under 1s (after 3 doses) 85% (95% CI 54% to 95%), 1 to 2 years 84% (61% to 94%), 3 to 4 years 99% (92% to 100%), 5 to 7 years 100% (72% to 100%), 8 to 10 years 92% (48% to 99%); 11 to 16 years 95% (88% to 98%) and 17 to18 years 92% (79% to 98%). Percentage reduction attack rates in unimmunised individuals are; for 2-4 yrs 70%, 5-8yrs 58%, 9-10 yrs 23%, 11-14 yrs 80% and 15-17 yrs 66%.

To determine the serum bactericidal antibody titre (using baby rabbit complement) (rSBA) that indicates protection, the predicted vaccine efficacy estimates for various rSBA cut-offs were compared with the observed efficacy and its 95% confidence intervals (95% CI) as calculated using the screening method. The rSBA titre correlating with protection was 1:8.

Based on the antibody persistence data from the clinical trials SBA levels in the youngest age groups are currently declining to near baseline so vaccinees will be reliant on immunological memory for protection. However, efficacy estimates remain high in toddlers. Reductions in serogroup C disease in unvaccinated individuals both in the cohorts targeted for immunisation and those over 20 yrs are consistent with herd immunity.

To assess the effect of MCC vaccines on carriage a multi-centre study was initiated based around 8 sampling centres (Glasgow, London, Nottingham, Oxford, Plymouth, Stockport, Bangor and Cardiff), The study design comprised 3 annual cross-sectional surveys of school students aged 15 to 19 years of age. In 1999, 2000 and 2001, 15,206, 18,273 and 19,755 students were sampled, respectively. For the 15 to 17 year old age cohort, from 1999 (pre MCC) to 2000 (post MCC) there was a 66% reduction in the carriage of serogroup C meningococci (p = 0.004). A further decline in serogroup C carriage was seen 2000 to 2001. These results are consistent with MCC vaccines protecting against carriage of serogroup C meningococci for at least 2 years.

Meningococci can change serogroup while maintaining all other clonal features for example the switch from serogroup B to C of the hypervirulent ET37 clone in 1960s. Concerns have been expressed that ‘vaccine pressure’ as a result of the U.K. serogroup C conjugate vaccine programme could cause a shift back from C to B or to other serogroups. To investigate whether capsular switching might occur, a subcapsular marker for the serogroup C ET37 (ST 11) complex was chosen. This subcapsular marker was the PorB serotype 2a (and confirmed by Multi Locus Sequence Typing). No evidence of capsular switching has been seen to date.