Sources of Airborne Norovirus in Hospital Outbreaks

BACKGROUND: Noroviruses are the major cause of viral gastroenteritis. Disease transmission is difficult to prevent and outbreaks in healthcare facilities commonly occur. Contact with infected persons and contaminated environments are believed to be the main routes of transmission. However, noroviruses have recently been found in aerosols and airborne transmission has been suggested. The aim of our study was to investigate associations between symptoms of gastroenteritis and presence of airborne norovirus, and to investigate the size of norovirus carrying particles.

METHODS: Air sampling was repeatedly performed close to 26 patients with norovirus infections. Samples were analysed for norovirus RNA by RT-qPCR. The times since the patients' last episodes of vomiting and diarrhoea were recorded. Size separating aerosol particle collection was also performed in ward corridors.

RESULTS: Norovirus RNA was found in 21 (24%) of 86 air samples from 10 different patients. Only air samples during outbreaks, or before a succeeding outbreak, tested positive for norovirus RNA. Airborne norovirus RNA was also strongly associated with a shorter time period since the last vomiting episode (odds ratio 8.1, p=0.04 within 3 hours since the last vomiting episode). The concentration of airborne norovirus ranged from 5-215 copies/m3, and detectable amounts of norovirus RNA were found in particles <0.95 µm and >4.51 µm.

CONCLUSIONS: The results suggest that recent vomiting is the major source of airborne norovirus and imply a connection between airborne norovirus and outbreaks. The presence of norovirus RNA in submicrometre particles indicates that airborne transmission can be an important transmission route.