Norovirus Attack Rates in a Long-Term Care Facility
A 120-bed long-term care facility reports 42 residents and 8 staff with acute vomiting or diarrhea over five days. The facility has two dining rooms: residents in Wing A eat in Dining Room 1 (60 residents; 30 ill) and Wing B eat in Dining Room 2 (60 residents; 12 ill). A food handler who prepared salads for both dining rooms was ill on the first day but worked. Staff cases include nurses and dietary aides. The administrator wants to close Dining Room 1 and exclude all ill staff for 72 hours. The local health department asks for a line list and attack rates. Two residents with symptoms had negative stool cultures for bacterial pathogens; no norovirus testing has been done. The facility’s hand hygiene audit shows 55% compliance among staff. Several residents in Wing A have dementia and attend group activities.
1. Calculate the attack rates by dining room and interpret whether dining location explains the outbreak.
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Overall resident attack rate = 42/120 = 35%. Dining Room 1: 30/60 = 50%; Dining Room 2: 12/60 = 20%; risk difference = 30 percentage points, crude relative risk = 2.5. Staff attack rate = 8/30 = 26.7%. The crude association suggests Dining Room 1, but it is confounded by wing: Wing A residents may be more mobile, attend group activities, or have more staff contact. A food handler who worked while ill could produce a common-source pattern affecting both rooms, depending on which foods were served. To interpret, stratify by wing and compare within-wing attack rates, and collect food-specific exposures. If the difference persists within wings and is linked to a specific food, a foodborne source is more likely; if not, person-to-person transmission with crowding is more likely. The staff cases and 55% hand hygiene compliance support widespread transmission.
2. What case definition and study design would you use?
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Use a clinical case definition: acute onset of vomiting or diarrhea (at least three loose stools in 24 hours) in a resident or staff member between the first and last illness dates, with laboratory confirmation where available. Because the facility has a defined population, a retrospective cohort study is more efficient than a case-control study. Build a line list with demographics, wing, room, dining room, group activities, food items, staff contacts, onset date, symptoms, and specimen results. Calculate attack rates by exposure and relative risks; use stratified analysis or multivariable regression for confounding. Collect stool specimens from early cases for norovirus PCR; bacterial cultures are less useful now. Interview the ill food handler and review kitchen logs. The goal is to generate and test hypotheses, not just count cases.
3. Should the facility close Dining Room 1 and exclude ill staff for 72 hours? What measures are supported?
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Closing Dining Room 1 alone is unlikely to control norovirus because transmission is also person-to-person and via fomites; however, it may reduce crowding and shared serving. I would not rely on it as a single measure. Exclude ill staff until at least 48 hours after symptoms resolve, not a fixed 72 hours; the administrator’s 72-hour rule may be too short for some and unnecessarily long for others. Cohort ill residents, use contact precautions, and perform hand hygiene with soap and water because alcohol is less effective against norovirus. Clean high-touch surfaces with bleach. Stop group activities during the outbreak. Notify the health department and communicate with families. Monitor for new cases for at least 48 hours after the last case.