Josephine van Dongen

2 Acute gastroenteritis disease burden among infants with medical risk conditions 27 Introduction Acute gastroenteritis (AGE) is a common infection with the highest incidence in young children. 1,2 The large majority of childhood AGE episodes are caused by enteric viruses, with rotavirus and norovirus being the dominant pathogens 1,3 . Premature bir th (<36 weeks of gestation), low bir thweight (<2500 grams) and/or presence of a congenital disorder are known risk factors for severe and complicated AGE, which is reflected in increased hospitalization rates, prolonged hospital stay and mor tality compared to healthy infants 4–8 . As AGE is mainly a self-limiting disease, the majority of episodes occur outside of the hospital setting 9 . To quantify the disease burden in the community, several observational studies in Europe have evaluated AGE incidence and healthcare attendance among healthy children, adults or elderly and estimated the contribution of various enteric pathogens 10,11 . There is reason to assume that the community AGE burden is increased among infants with medical risk conditions, similar to what is observed for AGE hospitalizations. However, no studies have specifically addressed community AGE among medical risk infants. Such data on specific risk- groups are valuable to prioritize target-groups for preventive interventions and assess the cost-effectiveness of various vaccination strategies against rotavirus and, possibly in the future, norovirus 12,13 . Our aim is to quantify the all-cause AGE and virus specific community burden of disease in medical risk infants in the Netherlands, and to identify infants most at risk of severe disease. Materials and methods Study design The Netherlands has not yet implemented rotavirus vaccination in the infant national immunization program (NIP). Uptake of rotavirus vaccine in the private market is less than one percent 14. This prospective cohor t study is par t of the Risk group Infant Vaccination Against Rotavirus (RIVAR) project. In brief, RIVAR pilots the implementation of a selective rotavirus vaccination program for medical risk infants organized in secondary pediatric care. Thir teen Dutch hospitals that host a Neonatal Intensive Care Unit or a neonatal post High/Intensive Care ward par ticipated in the pilot and implemented rotavirus vaccination between May 2016 and October 2017. Implementation was combined with a before-after cohor t study. All cohor t par ticipants were followed for the occurrence of AGE from enrollment between 6-14 weeks until 18 months of age.The current study uses data from the pre-implementation cohor t only, recruitment ran from November 2014 to October 2017, when rotavirus vaccination was not yet routinely available in the hospitals.