Lessons from a large norovirus outbreak: impact of viral load, patient age and ward design on duration of symptoms and shedding and likelihood of transmission.

BACKGROUND: Hospital norovirus outbreaks cause significant financial and operational disruption which should be minimised by optimal handling of affected areas and use of isolation facilities. AIM: To identify factors associated with increased duration of symptoms and viral excretion and increased probability of transmission.
METHODS: Retrospective observational study of a large norovirus outbreak at a UK teaching hospital in the winter of 2009-2010 where patients were diagnosed using a real-time polymerase chain reaction (PCR) assay.
FINDINGS: Symptom duration was significantly associated with patient age (Spearman rank correlation coefficient: 0.197; P = 0.002) but not with PCR cycle threshold (C(T)) value. Duration of viral excretion was found to be longer in patients with higher viral loads. Transmission within a ward bay was not significantly associated either with age or with C(T) value but was more likely to occur in some ward blocks than others, which may relate to differences in ward design. Transfer of patients into isolation rooms or cohorted area within two days of symptom onset did not significantly influence probability of onward transmission (52% vs 47%; P = 0.67).
CONCLUSIONS: The presented data suggest that C(T) value may guide timing of repeat sample collection if ongoing gastrointestinal symptoms may relate to other pathologies, and that patients developing symptoms of norovirus may remain in their current bay rather than being moved into isolation facilities. The bay or ward should be closed to new admissions but it should be anticipated that duration of symptoms and therefore closure will be longer when the outbreak involves elderly patients.