Epidemiology of Foodborne Norovirus Outbreaks – United States, 2009–2015

Noroviruses are the leading cause of acute gastroenteritis and foodborne disease in the United States (U.S.). About 1 in 5 reported norovirus outbreaks are spread through foodborne transmission, presenting opportunities for prevention. We describe the epidemiology of U.S. foodborne norovirus outbreaks reported to national surveillance systems, including differences between genotypes. Foodborne outbreaks that occurred during August 2009–July 2015 with norovirus reported as a single confirmed etiology to the National Outbreak Reporting System (NORS) were matched with outbreaks reported to CaliciNet, a U.S. laboratory norovirus outbreak surveillance network. We analyzed these matched outbreaks stratified by genotype for epidemiologic characteristics, including setting, size and duration, health outcomes of case-patients, implicated food, and outbreak contributing factors. Four hundred ninety-three confirmed foodborne norovirus outbreaks were reported in both NORS and CaliciNet. The most common norovirus genotypes reported were GII.4 (52%), GII.6 (9%), and GI.3 (8%). Compared to non-GII.4 outbreaks, GII.4 outbreaks had higher hospitalization rates (12.8 vs. 4.8 per 1,000 cases, P < 0.01). While contaminated foods were identified and reported in only 35% of outbreaks, molluscan shellfish (4% overall) were more often implicated in non-GII.4 outbreaks than in GII.4 outbreaks (7% vs. 1%, P = 0.04). Of the 240 outbreaks reporting at least one contributing factor, food workers were implicated as the source of contamination in 182 (76%), with no difference between GII.4 and non-GII.4 (73% vs 79%, P = 0.3). Foodborne norovirus outbreaks are frequently reported in the U.S., most of which are caused by GII.4 noroviruses. Viruses of this genotype are associated with higher rates of hospitalization; non-GII.4 noroviruses are more frequently associated with contaminated molluscan shellfish. These surveillance data highlight the diversity of noroviruses causing foodborne disease and can help guide appropriate food safety interventions, including worker hygiene, improved food handling and preparation, and further development of norovirus vaccines.