The purpose of this study is to assess the immunogenicity, safety and immune persistence of recombinant trivalent rotavirus subunit vaccine in healthy infants aged 6-12 weeks and healthy toddlers aged 7-71 months.

This clinical trial is aimed to evaluate the immunogenicity, safety and immune persistence of recombinant trivalent rotavirus subunit vaccine in Chinese healthy infants aged 6-12 weeks and healthy toddlers aged 7-71 months.The subjects will be divided into 12 subgroups. Two different immune regimens and two dose levels will be evaluated in each age group. Toddlers aged 7-71 months will receive two intramuscular injections on Day 0 and 28 or three intramuscular injections on Day 0, 28 and 56. Infants aged 6-12 weeks will receive three intramuscular injections on Day 0, 28 and 56 or Day 0, 56 and 112. Two dose (mid dose and high dose) will be included in each age group. To maintain blindness in the trial, in each age group with fixed immune regimen, subjects will be randomized in a 1:1:1 ratio to receive mid dose vaccine, high dose vaccine, or placebo.

Inclusion Criteria:

1. Healthy infants aged 6-12 weeks and healthy toddlers aged 7-71 months;
2. Legally acceptable representative (guardian) properly informed about the study and having signed the informed consent form (ICF).

Exclusion Criteria:

First dose exclusion criteria:

1. Axillary temperature >37.0℃ before vaccination;

2. Recepit of any rotavirus vaccine in the past;

3. History of intussusception or suffering from intussusception or history of any chronic gastrointestinal disease, including congenital malformations of the gastrointestinal tract that are likely to cause intussusception (such as Meckel's diverticulum);

4. Congenital malformations, developmental disorders, genetic defect, severe malnutrition, etc.;

5. Subjects aged 2 years or younger with history of dystocia, suffocation rescue, or nervous system damage;

6. Subjects aged 2 years or younger with history of premature birth (<37 weeks' gestation) or low birth weight (weight at birth of<2500 g);

7. History of convulsions, epilepsy and cerebral palsy, or mental illness and family history;

8. History of severe anaphylactic reaction to vaccination, or allergy to any components of the study vaccine;

9. Acute diseases (such as fever>39.0℃) or acute exacerbation of chronic disease within 3 days before vaccination;

10. Receipt of immune enhancement (including oral or intravenous immunoglobulin, but hepatitis B immunoglobulin is acceptable) or immunosuppressive therapy (continuous oral or intravenous infusion for more than 14 days) within 3 months;

11. Recepit of live attenuated vaccines within 14 days, or other vaccines within 7 days;

12. Congenital or acquired immunodeficiency, HIV infection, lymphoma, leukemia, systemic lupus erythematosus (SLE), juvenile rheumatoid arthritis (JRA), or other autoimmune diseases;

13. History of coagulation abnormalities (such as lack of blood coagulation factors, blood coagulopathy);

14. Primary and secondary impairment of immune function (history of thyroid, pancreas, liver, spleen resection, or treatment due to thyroid disease within the past 12 months);

15. Concurrent participation or plan to participate in another clinical trial throughout the study;

16. According to the judgment of the investigator, the subject has any other factors that are not suitable for participating in the clinical trial.

    Subsequent vaccination exclusion criteria:

17. Severe allergic reaction after the previous injection of study vaccine;

18. Serious adverse reactions that are causally related to the previous vaccination;

19. After the first vaccination, subjects with newly discovered or newly happened diseases that meet the first dose exclusion criteria will be determined by the investigator whether to continue participating the study;

20. Other reasons for exclusion judged by the investigator.