Circumvention of herd immunity during an outbreak of meningococcal disease could be correlated to escape mutation in the porA gene of Neisseria meningitidis.

Meningococcal strains isolated during an outbreak were shown to belong to the ET-5 complex and to harbor a mutation in the VR2 region of the porA gene. They were less susceptible to the bactericidal effect of normal human serum than was the ET-5 wild-type strain. These results are of concern, as PorA is a potential target in vaccine design.