Community Incidence of Norovirus-associated Infectious Intestinal Disease in England: Improved Estimates Using Viral Load for Norovirus Diagnosis

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Abstract
Existing estimates of the incidence of infectious intestinal disease (IID) caused by norovirus 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, and RT-PCR has poor diagnostic specificity. In this study, viral load measurements were used to identify cases of norovirus-associated IID and to produce new incidence estimates for England. IID cases were ascertained in the Study of Infectious Intestinal Disease in England (1993–1996), and stool specimens were tested by semiquantitative real-time RT-PCR for norovirus. The age-adjusted community incidence of norovirus-associated IID was 4.5/100 person-years (95% credibility interval: 3.8, 5.2), equating to 2 million episodes/year. Among children aged less than 5 years, the community incidence was 21.4/100 person-years (95% credibility interval: 15.9, 27.7), and the incidence of consultations to general practitioners for norovirus-associated IID was 3.2/100 person-years (95% credibility interval: 2.6, 3.8), with 100,000 children visiting their general practitioner for norovirus-associated IID each year. Norovirus is the most common cause of IID in the community in England and is responsible for a similar number of pediatric primary care consultations as rotavirus.