A comprehensive prevention strategy that attempts to address all possible routes of norovirus transmission and a strict ill employee exclusion policy are key to halting successive norovirus outbreaks, according to report results published in the Morbidity and Mortality Weekly Report.

In order to establish a cause, assess possible transmission routes, and provide control recommendations for a gastrointestinal illness outbreak at a single event center in Nebraska, researchers conducted a case-control study of 9 events occurring at the facility between October 27, 2017 and November 18, 2017. Employees and event attendees who experienced diarrhea (≥3 loose stools within 24 hours) or vomiting and at least one other symptom (eg, nausea, or abdominal cramps) were considered a probable case. Controls were defined as employees or event attendees who were not ill and attended an event during the study period. An internet-based questionnaire was sent to employees and event attendees to assess symptom history, events attended, and food items consumed.

Overall, 159 cases consistent with norovirus infection (156 probable and 3 confirmed) were identified. Estimated attack rates for the first 6 events, which occurred before any public health intervention, ranged from 7% to 35% per event (median, 18.5%).

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The investigation revealed a public vomiting episode in the carpeted lobby at the entrance of the facility on October 27. On November 7, investigators learned that there was inadequate sanitization of this area and recommended “sanitizing environmental surfaces with a sodium hypochlorite (chlorine bleach) solution or a disinfectant specifically registered by the Environmental Protection Agency as effective against norovirus and excluding ill employees from work until ≥48 hours after symptom resolution.”

Despite this recommendation, cases of gastroenteritis occurred at 2 events on November 10 (event 7) and November 11 (event 8). Event 7 and 8 indicated ongoing transmission with an estimated attack rate of 4% (6 of 150 attendees) and 15% (53 of 360 attendees), respectively. Moreover, investigators learned that an employee who became ill with nausea, vomiting, fever, headache, and myalgias on November 7, returned to work preparing and serving food less than 24 hours later.

Real-time reverse transcription-polymerase chain reaction detected norovirus genogroup II from the stool samples of the 3 ill individuals. Further genetic sequencing conducted by the Nebraska Public Health Laboratory and the United States Centers for Disease Control and Prevention yielded the same results. Two of the cases with laboratory-confirmed norovirus attended the October 27 event (event 1) and the third attended the event on November 11 (event 8).

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With event 7 and 8 indicating ongoing transmission, the facility hired a professional cleaning service experienced with norovirus eradication on November 16 and 17. After sanitation and strict employee exclusion were enacted, the event held on November 18 (event 9) had an estimated attack rate of 1% (3 of 350 attendees). Event 9 indicated reduced transmission with no further illnesses reported to public health officials.

Although no testing of environmental surfaces was conducted, “inadequate sanitizing of the [carpeting at the entrance of the facility] and aerosolization of the virus resulting from subsequent vacuuming could both have led to further spread [of norovirus],” concluded the investigators. “Mitigation efforts for ongoing norovirus outbreaks in similar settings should include a comprehensive prevention strategy that addresses all possible routes of norovirus transmission,” they added. 


Free RJ, Buss BF, Koirala S, et al. Successive norovirus outbreaks at an event center – Nebraska, October-November, 2017. MMWR Morb Mortal Wkly Rep. 2019;68(28):627-630.

This article originally appeared on Infectious Disease Advisor