Investigation of an outbreak of hypersensitivity-type reactions during the 2004 national measles-mumps-rubella vaccination campaign in Brazil

► High rate of HAEs occurred during Brazil's National MMR vaccination campaign (2004). ► Dextran-contained MMR vaccine was associated with this higher reporting rate. ► Clinical findings suggested IgG and immune complex-mediated Type III reaction. ► Case-control failed to associate HAE cases with previously published risk factors. ► Studies subsequently implicated dextran as the likely cause of these events.