This is a proposal for a nested case-control study within an ongoing rotavirus vaccine immunogenicity clinical trial Karachi, Pakistan. The primary study aim is to compare the fecal microbiota composition and diversity of infants who do (control) and do not (case) demonstrate immune seroconversion to rotavirus vaccination. The infants will be matched for vaccination dose, age and breast-feeding practices.

Inclusion Criteria:

• 6 weeks 0 days to 7 weeks 6 days age at the time of enrollment.
• Healthy infant free of chronic or serious medical condition as determined by history and physical exam at time of enrollment into in the study.
• Written informed consent obtained from the parents or guardians.
• Availability of baseline fecal sample collected before Rotarix vaccination
• Written informed consent obtained from the parents or guardians for nested study

Exclusion Criteria:

• Hypersensitivity to any of the vaccine components
• Use of any investigational drug or vaccine other than the study vaccine within 30 days of first dose of study vaccine or during the study.
• Use of any immunosuppressive drugs.
• Previous intussusceptions or abdominal surgery.
• Enrollment in any other trial (besides NCT01199874).
• Birth weight less than 1500 grams; or if birth weight is unknown, weight less than 2000 grams on or before 28 days.
• Immunoglobulin and/or blood products use since birth or during the study period. Nested study additional exclusion criteria:
• Positive serum anti-rotavirus Immunoglobulin A (> 20 U/ml) at 6 weeks of age, indicative of prior rotavirus infection

The study will be nested within an existing rotavirus immunogenicity study being conducted in Karachi, Pakistan (clinicaltrials.gov: NCT01199874) . The physical setting is a peri-urban slum outside of Karachi, primarily populated by fishermen. Parents and legal guardians of infants participating in NCT01199874 will provide new informed consent for inclusion in this study.