Towards measles elimination in Italy: monitoring herd immunity by Bayesian mixture modelling of serological data.

The analysis of post-vaccination serological data poses nontrivial issues to the epidemiologists and policy makers who want to assess the effects of immunisation programmes. This is especially true for infections on the path to elimination as is the case for measles. We address these problems by using Bayesian Normal mixture models fitted to antibody counts data. This methodology allows us to estimate the seroprevalence of measles by age and, in contrast to conventional methods based on fixed cut-off points, to also distinguish between groups of individuals with different degrees of immunisation. We applied our methodology to two serological samples collected in Tuscany (Italy) in 2003 and in 2005-2006 respectively, i.e., before and after a large vaccination campaign targeted to school-age children. Besides showing the impact of the campaign, we were able to accurately identify a large pocket of susceptible individuals aged about 13-14 in 2005-2006, and a larger group of weakly immune individuals aged about 20 in 2005-2006. These cohorts therefore represent possible targets for further interventions towards measles elimination.