Presymptomatic Contagiousness of Norovirus
People infected with norovirus can shed virus and potentially transmit infection before symptoms begin, though the exact duration of presymptomatic contagiousness remains incompletely defined—one documented case showed viral shedding one day before symptom onset. 1
Evidence for Presymptomatic Transmission
The most direct evidence comes from a study of elderly patients where norovirus was detected in stool samples collected one day before gastroenteritis symptoms began, demonstrating prodromal viral excretion. 1 This represents the clearest documentation of presymptomatic shedding in the literature.
However, the broader epidemiologic picture is less certain:
- Presymptomatic shedding has been suspected on epidemiologic grounds but was not proven in volunteer studies conducted through 1990. 2
- The typical incubation period is 12-48 hours after exposure before symptoms develop. 3, 4
- Given that viral particles can be detected before symptom onset, transmission during this window is biologically plausible and has been observed in outbreak investigations. 1
Clinical Implications for Infection Control
The detection of presymptomatic viral shedding means that infection control measures cannot rely solely on symptomatic individuals:
- Infected persons may be contagious during the 24-48 hours before symptoms appear, based on the incubation period and documented presymptomatic shedding. 4, 1
- This is particularly important in closed environments like nursing homes, cruise ships, and healthcare facilities where asymptomatic or presymptomatic individuals can serve as unrecognized sources of transmission. 2
- Up to 30% of norovirus infections are completely asymptomatic, yet these individuals can still shed virus at lower titers, further complicating outbreak control. 4
Peak Contagiousness Timeline
While presymptomatic shedding occurs, the highest risk period for transmission is different:
- Peak viral shedding occurs 2-5 days after infection (which corresponds to the early symptomatic period), with approximately 100 billion viral copies per gram of feces. 4
- Viral shedding continues for an average of 4 weeks following infection, though infectivity beyond the acute phase remains unclear. 3, 4
- In elderly patients specifically, the median excretion time was 8.6 days (range 2-15 days) in one study. 1
Common Pitfalls in Outbreak Management
- Do not assume that only symptomatic individuals are contagious—presymptomatic and asymptomatic shedding both occur. 4, 1
- Formed stools can contain infectious norovirus, so infection control precautions should not be relaxed based on stool consistency alone. 1
- The extremely low infectious dose of 10-100 viral particles means even minimal presymptomatic shedding can cause transmission. 4
- Alcohol-based hand sanitizers have limited efficacy against norovirus—handwashing with soap and water for at least 20 seconds is essential. 3