The primary objective is to measure the effect of host human genetics on the resulting immunological responses and long-term protection following rotavirus immunization of a study population of infants in Ho Chi Minh City, Vietnam.

The secondary objectives are to assess the temporal immunological responses following rotavirus vaccination, and to investigate the role of maternally derived antibodies, and other factors that could potentially affect immunological responses following rotavirus vaccination. Also to assess infecting rotavirus genotypes in the vaccine failure cases.

This study will be an open single arm, single centre, interventional study conducted in Ho Chi Minh City, Vietnam. A sample size of 1,000 infants will be recruited to the study between 8 to 9 weeks (which is around 2 months) of age. These infants will receive two doses of the rotavirus vaccine, Rotarix with an interval of 28-37 days (or 4-5 weeks). The parents will be requested to bring the infants 2-3 days following each Rotarix vaccination to receive the standard EPI vaccinations. The infants will then be under passive and active surveillance for rotavirus-associated diarrhoea until 18 months of age. Blood samplings during pre and post vaccination and at 6, 12 and 18 month old visits will be collected.

Inclusion Criteria:

• Infants (both males and females) between 8 and 9 weeks (i.e. approximately 2 months) of age who are attending Hung Vuong Hospital to receive their standard EPI vaccinations.
• Currently registered residents of Districts 5 (where Hung Vuong Hospital is located) or 1, 6, 8, 10, or 11 (districts that are neighbouring district 5) in Ho Chi Minh city, with no specific intention of relocating in the next 12 months.
• Informed consent to be enrolled in the study and comply with study procedures, including host genetic studies.

Exclusion Criteria:

• Refusal of consent.
• Parent/ guardian under the age of 18.
• Premature (i.e. gestation period <37 weeks).
• Infants who have already been immunized with either a rotavirus vaccine or the standard 2-month EPI vaccinations.
• History of hypersensitivity to any components of the vaccine or adverse vaccine event.
• History of intussusception or congenital malformation of the gastrointestinal tract in the child that is likely to predispose child to intussusception.
• History of severe combined immunodeficiency disease (SCID), acquired immune deficiency syndrome (AIDS) or Human immunodeficiency virus (HIV) positivity, or other known immunodeficiency syndromes that may place the child at risk during immunisation.