Vomiting calls to NHS Direct provide an early warning of norovirus outbreaks in hospitals

A wintertime peak of norovirus activity occurs each year, affecting institutions including schools and hospitals. Traditional laboratory and outbreak surveillance systems for norovirus are too vulnerable to reporting delay to act as a timely signal of activity in the community. Calls to the National Health Service (NHS) telephone service NHS Direct have the potential to be an early warning tool for public health purposes. We investigated whether NHS Direct vomiting calls can be used as a reliable indicator of norovirus activity and, if so, whether the increase in calls precedes the epidemic of hospital outbreaks. Laboratory reports were used as the reference standard to define the norovirus season. From 2004 to 2008, four series of NHS Direct call data were compared with laboratory data held at the Health Protection Agency Centre for Infections in order to identify the best predictor of the season start. The four series included: (1) modelled and extracted the proportion of calls likely to be for 'non-rotavirus' gastroenteritis; (2) the mean proportion of weekly vomiting calls in children aged <5 years; (3) the mean proportion of weekly vomiting calls for all ages; (4) the slope of the vomiting call data. Issuing an alert when 4% or more of NHS Direct vomiting calls in all age groups for two weeks in a row should provide up to four weeks' advance warning of forthcoming norovirus pressures on the health service.