Model-based comparisons of measles immunization strategies using high dose Edmonston-Zagreb type vaccines

The World Health Organization has recently made a new recommendation for measles immunization in areas with very high rates of transmission. The new policy will involve a switch to the high potency Edmonston-Zagreb (E-Z) vaccine (a strain with improved efficacy in the presence of maternally-derived, specific antibody), targeting immunization at six-month-old infants. We present a mathematical model of measles transmission that can be used to assess the likely impact of cohort immunization on the incidence of infection using the new strain of vaccine. Data requirements of the model are discussed, with special reference to the critical issue of age-specific vaccine efficacy. Currently available information on seroconversion rates achieved with the new strain are summarized. Results are presented which endorse the new WHO recommendations, predicting considerable benefits, in particular, to those most at risk of measles morbidity and mortality. Furthermore, integration of E-Z with the third diphtheria, pertussis, tetanus (DPT) dose at three-four months should be considered as an alternative option, particularly if combined with some additional coverage at nine months of age.