A new approach to the definition of seroconversion following vaccination in a population with high background antibody concentrations.

Licensure of meningitis vaccines is increasingly being made on the basis of safety and immunogenicity. For meningococcal vaccines, a measure of immunogenicity is seroconversion, usually defined as a four-fold increase in rSBA titre. However, this definition is likely to underestimate seroconversion in settings with high background immunity. Using data from a study of the immunogenicity of meningococcal polysaccharide vaccines undertaken in Ghana, a logistic regression model to estimate the probability of seroconversion as a function of pre-vaccination titre has been developed. The seroconversion rate (91%) based on a variable-fold increase in rSBA titre derived from the model was a more plausible estimate of immunogenicity than the seroconversion rate (32%) based on the fixed four-fold increase in rSBA.