Rotavirus is the commonest cause of severe gastroenteritis in young children, and premature babies are at high risk of serious disease. The RV1 rotavirus vaccine is an altered form of the virus and is given to all children at 8 weeks and 12 weeks of age in the UK. It is given by mouth and excreted in the stool following vaccination - the vaccine virus can therefore be transmitted between people in the same way as the normal virus. Premature babies often receive their initial vaccines while they are still hospitalised in neonatal units. There are no data regarding stool excretion of the RV1 rotavirus vaccine in premature babies - there is a theoretical risk of transmission of the vaccine virus to other babies in the unit (if there was inadequate staff hand washing). Some countries have therefore advised against immunisation of babies while in the neonatal unit. In the UK, however, the vaccine is being given in neonatal units, enabling evaluation of the vaccine in hospitalised premature babies.