During a waterborne outbreak of norovirus in Spain, we estimated 50% illness doses for a group of exposed (secretor) persons to be 556 (95% CI 319-957) genome copies/day for norovirus GI and 2,934 (95% CI 1,683-5,044) genome copies/day for norovirus GII. Use of a propidium monoazide viability assay reduced these values.
Keywords: 50% illness dose; GI; GII; Spain; bottled mineral water; dose causing illness; genotypes; infectivity; norovirus; real-time quantitative PCR; secretor status; viruses; water cooler; waterborne outbreak.