title: Unravelling the epidemiology of norovirus outbreaks in hospitals
Creators: name: Harris, John
Publisher: name: University of Liverpool Repository
Year: 2014

description: Norovirus is the commonest cause of outbreaks of gastrointestinal disease in the U.K. Most reported outbreaks occur in health care settings, such as hospitals and nursing homes, and can cause severe disruption through ward closures, cancelled operations and staff sickness. Previous studies estimated these outbreaks cost the NHS around £115 million a year. Despite previous studies some questions remain. What is the burden of norovirus in hospitals - how many outbreaks occur and how many people are hospitalised each year as a result of norovirus infection? Do published reports of outbreaks provide evidence of what works in infection control? Can the factors facilitating norovirus transmission during outbreaks in hospitals be identified? These questions were answered through a series of inter-linked studies that explored mortality, morbidity, transmission pathways and aspects of infection control. The introduction of a new surveillance system provided greater insights into the heavy burden that norovirus imposes on English hospitals. In the years 2009-2011, 3,980 reports of outbreaks of suspected and confirmed norovirus were received. There was little difference in the epidemiology of outbreaks from one season to the next. On average outbreaks were associated with 13,000 patients and 3,400 staff becoming ill, 8,900 days of ward closure and the loss of over 15,500 bed-days annually. Analysis of mortality data demonstrated a clear association between norovirus infection and mortality in the elderly (65 years and over) with an estimated 80 deaths per year in this age group. The number of deaths increased in years where norovirus activity was higher but this was not associated with increased pathogenicity of the virus. Norovirus was the only pathogen that had a significant association with mortality in the regression models. Modeling of routine hospital admission data demonstrates that norovirus accounted for around 3,000 norovirus admissions a year to English hospitals, two thirds of which were in the elderly. A review of published papers did not provide clear evidence for the effectiveness of infection control measures. However, this was largely because the reporting of outbreaks was poor and that the introduction of more rigorous reporting protocols would improve this. Analysis of 3,500 outbreaks of norovirus demonstrated that closing a ward or bay promptly (within three days of the first person becoming ill) is beneficial. The duration of outbreak, the total duration of disruption were shorter, and fewer patients overall were affected, if closure occurred promptly. When closure occurred 7 or more days after the first onset date outbreaks were twice as long as those where closure was prompt. The duration of outbreak was also increased by ward size and in outbreaks occurring in winter time. Outbreaks were longer if they occurred on care of the elderly wards. A strategy of prompt closure is beneficial, particularly in larger wards and during winter time. The time between the first two cases of each outbreak was used to estimate the serial interval for norovirus in a hospital setting and was estimated to be 1.86 days. This distribution and dates of illness onset were used to calculate epidemic trees for each outbreak. A permutation test found strong evidence that proximity was a significant driver of outbreaks (p