In Guinea-Bissau, a combined OPV and VAS campaign took place in November 2002. Given the uncertainty about the best dose of VAS, we examined whether the half dose compared with the full dose currently recommended by WHO gave an equally good protection against childhood morbidity and mortality. Mortality after supplementation was lower, though not significantly so, for children who had received the half dose. However, there was a highly significant inversion of the effect for boys and girls; while the low dose was clearly better for girls, the full dose might have been slightly better for boys. The girls' responses to the high versus the low dose of vitamin A might have depended on the last vaccine received before the OPV and VAS campaign.