Eligible,healthy infants who have finished the 3-dose-schedule of sIPV+DTaP combined vaccination clinical trial (NCT04054882) will be recruited and divided into 4 groups, and will receive vaccination at the age of 18-month-old as follows:

Group 1: sIPV + DTaP + MMR, Group 2: sIPV only, Group 3: DTaP only, Group 4: MMR only.

The immunogenicity and safety of the 4 groups will be compared and analyzed before and 30 days after vaccination.

Following the clinical trial of "Combined Immunization of sIPV and DTaP" in 2019, this study recruits 600 18-month-old subjects who have received 3 doses of sIPV + DTaP, and gives them a 4th dose of vaccination (booster immunization). They are divided into 4 different groups, with 150 subjects in each group, and are innoculated with different vaccines.

To be specific, group 1 receives sIPV (0.5ml)+ DTaP (0.5ml)+ MMR(0.5ml); group 2 receives sIPV (0.5ml); group 3 receives DTaP (0.5ml); group 4 receives MMR (0.5ml).

Blood samples will be collected before vaccination and 30 days after this booster immunization. Neutralization antibody will be detected to evaluate the seroprotection rates and antibody geometric mean concentrations. The safety of both immunization schedule will be monitored as well.

Inclusion Criteria:

• Subjects must have participated the clinical trial titled "Clinic Trial to Evaluate the Safety and Immunogenicity of Combined Immunization of sIPV and DTaP" (NCT04054882) in 2019, and have finished 3 doses of combined immunization of sIPV and DTaP;
• Subjects aged 18 months old at the date of recruitment;
• With informed consent form (ICF) signed by parent(s) or guardian(s);
• Parent(s) or guardian(s) are able to attend all planned clinical appointments and obey/follow all study instructions;
• Subjects have been vaccinated with a first dose of MMR, but have not been vaccinated with the 2nd dose of MMR and the booster (4th) dose of sIPV and DTaP;
• No less than 14 days since the last dose of vaccination;
• Axillary temperature ≤37.0℃.

Exclusion Criteria:

• With a medical history with hypersensitiveness, eclampsia, epilepsy, cerebropathia and neurological illness;
• Allergic to any ingredient of vaccine or with allergy history to any vaccine;
• Subjects with immunodeficency or suspected impairment of immunologic function (e.g. caused by HIV), or subjects are in the process of immunosuppressor therapy(Taking orally injecting of steroid hormone);
• Administration of immunoglobulins within 30 days prior to this study;
• Acute febrile disease(temperature ≥ 37.0°C) or infectious disease;
• With a clearly diagnosed history of thrombocytopenia or other coagulopathy, may cause contraindications for subcutaneous injection;
• With any serious chronic illness, acute infectious diseases, or respiratory diseases;
• With severe cardiovascular disease, liver and kidney diseases or diabetes mellitus with complications;
• With any kind of infectious, purulent, or allergic skin diseases;
• With any other factor that makes the investigator determines the subject is unsuitable for this study.