Seroprevalence of antibodies to serogroup C meningococci in England in the post-vaccine era

Objective To describe the population prevalence of serogroup C serum bactericidal and IgG antibodies in England one to five years after the introduction of meningococcal serogroup C conjugate (MCC) vaccines in 1999/ 2000. Methods Serum samples collected between 2000 and 2004 were obtained from the Health Protection Agency Seroepidemiology Unit, which collects residual sera from participating laboratories across the country to enable serological surveillance of the National Immunisation Programme. Serum bactericidal antibody (SBA) against serogroup C strain C11 was determined for 2,396 sera using a standardised complement-mediated SBA assay with baby rabbit complement. The age-specific percentage of individuals with SBA titres ≥ 8 was calculated, together with 95% confidence intervals (CI). Results were compared with a pre-vaccine (1996-1999) seroprevalence study. In addition, serogroup-specific IgG concentrations were determined in 2664 sera using a tetraplex IgG bead assay for serogroups A, C, W-135 and Y. The age- and serogroup-specific geometric mean concentrations (GMC) for IgG and 95% CI were calculated. The year of and age at sample collection was used to infer the likely timing and schedule of MCC immunisation for each individual. In under 20 year olds all samples were taken from 2001 onwards, after the end of the MCC catch-up campaign. Results The percentage of individuals with SBA titres ≥ 8 was higher in the post-vaccine era for all age groups targeted for MCC vaccination (0-24 years), compared to the seroprevalence in 1996-1999 (pre-vaccine) as shown in figure 1. In the youngest children (