The main aim of this study is to investigate cases of rotavirus gastroenteritis in Belgian children with opportunity to receive Rotarix™ to monitor the potential occurrence of genetic drifts (point mutations) in the vaccine strain and the occurrence of genetic shifts (re-assortments) between vaccine and naturally circulating wild-type strains in Belgium population after the introduction of Rotarix™. The study will also detect if there is any alteration in rotavirus pathogenicity conferred by re-assortment and if the mutated vaccine strain is still efficacious in preventing rotavirus gastroenteritis.