Meningococcal protein NMB 1870 has been described in the prior art. The inventors have found that NMB 1870 is an effective antigen for eliciting anti-meningococcal antibody responses, and that it is expressed across all meningococcal serogroups. Forty-two different NMB 1870 sequences have been identified, and these group into three variants. Serum raised against a given variant is bactericidal within the same variant group, but is not active against strains which express one of the other two variants i.e. there is intra-variant cross-protection, but not inter-variant cross-protection. For maximum cross-strain efficacy, therefore, the invention uses mixture comprising different variants of NMB 1870.
