To address these hypotheses, the proposed study will recruit a prospective cohort of 420 mother-infant pairs. These will be enrolled at the time of vaccination and followed for up to four years. Baseline immunological status will be ascertained and seroconversion rates determined a month after full immunization. Incident rotavirus gastroenteritis will be monitored in the vaccinated infants whenever episodes of diarrhoea occur; through this surveillance, the sero-strains of rotaviruses causing disease will be tracked over the four year period. Contributions of HIV infection both in mothers and infants, vitamin A and zinc deficiency, weight for age Z-scores as well as mid upper arm circumference will also be assessed. Knowledge gained from this study will inform future interventional trials on strategies to improve rotavirus vaccine effectiveness in the developing world.