The Phase II clinical trial of the oral hexavalent reassortant rotavirus attenuated live vaccine (Vero Cells) will be conducted in infants aged 6 to 12 weeks. This study will evaluate the immunogenicity and safety of the investigational vaccine in healthy infants through a randomized, double-blind, active-controlled trial.

The Phase II clinical trial is a randomized, double-blind, active-controlled study conducted in healthy infants to evaluate the immunogenicity and safety of the investigational vaccine. The investigational vaccine is available in both high-dose and low-dose formulations. The control vaccine is the orally administered pentavalent reassortant rotavirus attenuated live vaccine (Vero Cells) produced by Merck Sharp & Dohme Corp.

This study plans to recruit 400 infants aged 6 to 12 weeks. All participants will be randomly assigned to the low-dose investigational group, high-dose investigational group, and active-controlled group, respectively.The immunization schedule for both the investigational vaccine and controlled vaccine consists of three doses administered at 28-day intervals.

Blood samples will be collected at predefined time points to evaluate the immunogenicity of the investigational vaccine. Adverse events (AEs) will be collected for all participants from the first vaccination until 42 days after the last dose, while serious adverse events (SAEs) and adverse events of special interest (AESIs) will be monitored for 12 months.

A safety monitoring sub-cohort will be established, with stool samples collected daily for 14 days after each vaccination to assess vaccine virus shedding, duration and patterns of viral shedding, potential reassortment and reversion to virulence of the rotavirus vaccine strains

Inclusion Criteria:

1. Healthy infants aged 6 to 12 weeks.
2. The legal guardian(s) is/are capable of understanding and voluntarily signing the informed consent form.
3. The legal guardian(s) is/are willing and able to comply with all follow-up visits, sample collection, vaccination, and other study procedures.
4. Able to provide valid legal identification documents.

Exclusion Criteria:

1. Previous vaccination with any rotavirus vaccine.
2. History of rotavirus infection.
3. Gestational age <37 weeks or ≥42 weeks at birth.
4. History of dystocia, neonatal asphyxia requiring resuscitation, or neurological impairment at birth.
5. Known hypersensitivity to any vaccine component (e.g., urticaria, dyspnea, angioedema).
6. Current diarrhea, vomiting, or other gastrointestinal disorders; gastroenteritis or any acute/chronic illness exacerbation within 7 days prior to vaccination; ongoing antibiotic/antiviral therapy.
7. History of intussusception or chronic gastrointestinal diseases, including congenital malformations predisposing to intussusception (e.g., Meckel's diverticulum).
8. Congenital malformations, developmental disorders, genetic defects, severe malnutrition, malignancies, or significant chronic conditions (e.g., Down syndrome, diabetes, sickle cell anemia, neurological disorders, Guillain-Barré syndrome).
9. Autoimmune or immunodeficiency diseases (including but not limited to asplenia, functional asplenia, HIV infection).
10. Household members with immunodeficiency/immunosuppression or undergoing/scheduled for immunosuppressive/cytotoxic therapy.
11. Coagulation disorders (e.g., clotting factor deficiencies, platelet abnormalities).
12. Immunosuppressive therapy for ≥14 days post-birth (prednisone ≥2mg/kg/day or equivalent), immunomodulatory/cytotoxic therapy, or planned use during the study.
13. History of severe neurological/psychiatric disorders (e.g., epilepsy, non-febrile seizures, convulsions) or relevant family history.
14. Postnatal administration of immunoglobulins/blood products (except hepatitis B immunoglobulin) or planned use during the study.
15. Previous participation in other investigational drug/vaccine studies or planned use during this study.
16. Receipt of live-attenuated vaccines within 14 days or subunit/inactivated vaccines within 7 days prior to enrollment.
17. Axillary temperature ≥38.0°C within the past 3 days.
18. Fever on scheduled vaccination day (axillary temperature >37.0°C; measured ≥30 minutes post-feeding).
19. Currently or planning to participate in other vaccine or drug clinical trials.
20. Any other factors that the investigator deems unsuitable for participation in the clinical trial.