With the global effort to eradicate polio, national immunization days with oral polio vaccine (OPV) offer an additional opportunity to provide vitamin A. In Guinea-Bissau, a combined OPV and VAS campaign took place in November 2002. Given the uncertainty about the best dose of VAS, we aimed to examine whether the dose of vitamin A currently recommended by WHO or half this dose gives a better protection against childhood morbidity and mortality.