Phase IV Clinical Study of Sequential Vaccination of Sabin Strain and Wild Strain Inactivated Poliovirus Vaccine

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.