The epidemiology of norovirus in England diagnostics, incidence and transmission

Existing estimates of the incidence of infectious intestinal disease (IID) caused by norovirus in the community are based on electron microscopy or reverse transcription- polymerase chain reaction (RT-PCR). Neither method accurately represents norovirus disease burden: electron microscopy has poor diagnostic sensitivity, RT-PCR has poor diagnostic specificity. In this thesis, for the first time, viral load measurements were used to identify cases of norovirus-associated IID, to update the existing, electron microscopy-based estimates of norovirus disease burden in the community in England and to conduct a comprehensive analysis of risk factors for sporadic norovirus-associated IID. The prevalence and characteristics of asymptomatic norovirus infection in England were also described. Data for this work were taken from the Study of Infectious Intestinal Disease faecal specimen archive, which had been subject to semi-quantitative real- time RT-PCR norovirus testing. Finally, routine surveillance data and time-series regression modelling were used to produce an additional and independent estimate of the incidence of general practice consultations for norovirus-associated IID in England and Wales. Using viral load for norovirus diagnosis, the age-adjusted community incidence of norovirus-associated IID in England was 4.5 per 100 person-years (95% credibility interval: 3.8,5.2), equating to 2 million episodes per year between 1993 and 1996. Amongst children aged less than five years, the community incidence was 21.4 per 100 person-years (95% credibility interval: 15.9,27.7) and the incidence of consultations to general practitioners for norovirus-associated IID was 3.2 per 100 person-years (95% credibility interval: 2.6,3.8), with 100 000 children visiting their GP for norovirus- associated IID each year. The main risk factor for sporadic, community-acquired norovirus-associated IID was contact with a person with IID symptoms. This result indicates that reduction of person- to-person transmission would substantially decrease the burden of norovirus- associated IID in the community in England, e. g. through good hand hygiene and appropriate cleaning of environmental surfaces.