Detection of norovirus infections in Denmark, 2011-2018

Norovirus occur very frequently yet are rarely diagnosed. In Denmark, norovirus infections are not under surveillance. We aimed to collect and describe existing laboratory-based norovirus data. National norovirus laboratory data were collected for 2011-2018, including information on patient identification number, age and sex, requesting physician, analysis date and result. We defined positive patient-episodes by using a 30-day time window and performed descriptive and time series analysis. Diagnostic methods used, were assessed through a survey. We identified 15809 patient episodes (11%) out of 142648 tested patients with an increasing trend, 9366 in 2011 vs 32260 in 2018. This corresponded with a gradual introduction of PCR-analysis in the laboratories. The highest positivity rate was in patients aged <5 years (15%) or >85 years (17%). There was a large difference in test performance over five Danish geographical regions and a marked seasonal variation with peaks from December to February. This is the first analysis of national norovirus laboratory-data in Denmark. A future laboratory-based surveillance system may benefit public health measures by describing trend, burden and severity of seasons and possibly pinpoint hospital outbreaks.