National immunization rates indicate high vaccine coverage in Mesoamerica, but there is growing evidence that the most vulnerable groups are not being reached by immunization programs. Therefore, there is likely low effective vaccine coverage in the region, leading to persistent and growing health inequity. The planning phase of this project was from June to December 2009. The project will be conducted in the target populations which includes children under five, pregnant women, and women of child-bearing age from the most vulnerable populations within countries of the Mesoamerican region, as indicated geographically by a low human development index (HDI) and/or high prevalence of poverty at the municipal level and through the use of participatory methods to define poverty and vulnerability in local contexts. We defined three lines of action for vaccine-preventable disease interventions: 1) pilot projects to fill gaps in knowledge; 2) strengthening immunization policy; and 3) implementation of evidence-based practices. Health system strengthening through health equity is the central regional objective of the immunization workgroup. We hope to have a transformational impact on health systems so as to improve effective coverage, including vaccine and other integrated primary healthcare services.