Although not a blinded evaluation, the introduction of bias will be reduced in several ways. First, the vaccine teams located in the EPI centres that are completely separate from the clinical settings where clinicians will be conducting surveillance. Second, the clinicians are separated from those scientists conducting laboratory testing. Third, laboratory assays for rotavirus will be carried out in the laboratory in Dhaka using coded specimens and are not conducted at the time of illness (since they have little clinical value). Although a parent could inform the clinician as to his/her child's participation in the study, this is believed to be an unlikely or important source of bias for two reasons. One, most rotavirus gastroenteritis is not seen until later than 6 months after an infant's second dose of Rotarix, and there will be no active follow-up among parents after last receipt of Rotarix. Thus, there is a reduced likelihood that parents will remember that their child had received a vaccination targeted specifically against one type of severe diarrhoea. Second, Matlab Diarrhoeal Hospital and the community treatment centres have been caring for severe diarrhoea for many years and the entire community is already accustomed to attending the facilities for such illnesses. Thus, there is a reduced likelihood parents of vaccinated and unvaccinated infants are going to have differential diarrhoeal illness treatment-seeking behaviors.