The goal of this study is to compare the immunogenicity and safety of sIPV administered via subcutaneous and intramuscular injection routes

This is an open-labelled, randomized, controlled clinical trial. Totally 480 healthy infants of 2 months old (aged 56-84 days) are planned to be enrolled, and then randomized in a 1:1:1:1 ratio into four groups (group A, B, C, D). Group A-D will receive three doses of primary immunization against polio as per the 2wIPV+1sIPV (First 2 doses: wIPV. 3rd dose: sIPV), 1wIPV+2sIPV (1st dose: wIPV. 2nd & 3rd doses: sIPV), 3wIPV, 3sIPV vaccination regimen, with an interval of 28 days (+14 days) between doses. In each intervention group, two-thirds of the participants will receive the full-course vaccination via intramuscular injections, while the remaining one-third will receive it via subcutaneous injections.

For all the participants, the immediate reactions within 30 minutes after each dose of vaccination will be observed on study site. Guardians of participants will utilize the diary card to record adverse events (AEs) from the time of vaccination for 7 days after each dose of vaccination, and will utilize the diary card to record any AEs from 8 days up to 28 days after each dose of vaccination. SAEs arise from the time of vaccination up to 28 days after the last vaccination will be collected.

About 3.0 ml venous blood will be collected before the first vaccination and 28 days (+14 days) after the last vaccination. Neutralizing antibody (Nab) titer against polioviruses of three serotypes will be determined for immunogenicity evaluation.

Approximately 5.0 ml of venous blood will be drawn from the mothers to test for these disease infections. Alternatively, mothers may provide relevant test results obtained during pregnancy for these infections. This approach will enable the investigators to identify any potential perinatal transmission to the infant and offer appropriate guidance and management for both the mother and the child.

Inclusion Criteria:

1. Infants of 2 months old (aged 56-84 days).
2. Have a parent/legal guardian who has provided written informed consent after being fully informed about the study.
3. Be able to provide the vaccination records since birth.
4. The infant's mother is tested negative for HIV, syphilis, hepatitis B, and hepatitis C during or before the infant's enrollment to this study (test results obtained during pregnancy are acceptable, if provided).

Exclusion Criteria:

1. History of any polio vaccination.
2. History of severe allergic reaction to previous vaccinations or hypersensitivity to any vaccine component.
3. Premature infants (born before week 37 of gestation).
4. History of asphyxia rescue or nervous system injury.
5. Congenital malformations, developmental disorders, clinically significant genetic defects, severe malnutrition.
6. Autoimmune diseases or immunodeficiency/immunosuppression.
7. Serious chronic diseases such as Down's syndrome, diabetes, sickle cell anemia, or neurological disorders.
8. Abnormal coagulation functions (e.g., coagulation factor deficiency, blood coagulation diseases, platelet disorders).
9. Received immunosuppressant (excluding topical or inhaled corticosteroids), cytotoxic drug, or other immunomodulatory therapies.
10. Received blood products before trial vaccine inoculation.
11. Received other study drugs within 30 days before enrollment.
12. Received live attenuated vaccines within 14 days before enrollment.
13. Received subunit or inactivated vaccines within 7 days before enrollment.
14. Acute diseases or acute exacerbations of chronic diseases within 7 days before enrollment.
15. Significant acute diseases, chronic infections, or axillary temperature ≥ 37.5°C before enrollment.
16. Any other factors deemed by the investigators as unsuitable for participation.