This multicenter, randomized, double-blind, placebo-controlled phase IIIa clinical trial aims to evaluate the protective efficacy , immunogenicity and safety of the Tetravalent Inactivated Enterovirus Vaccine (Vero Cell) in Children aged 6 to 71 months.

Participants will be randomly assigned in a 1:1 ratio to the trial group and the placebo group, receiving two doses of experimental vaccine or placebo , with a one-month interval between the two doses.

This trial employs a multicenter, randomized, double-blind, placebo-controlled design.With informed consent, 6000 children participants aged 6 to 71 months will be enrolled, with 2800 participants aged 6 to 23 months and 3200 participants aged 24 to 71 months, maintaining a balanced gender ratio. All participants will be randomly assigned to the trial and placebo groups at a 1:1 ratio, receiving two doses of experimental vaccine or placebo , with a one-month interval between the two doses.

Protective Efficacy Assessment: Each participant enters the case monitoring period after receiving the first dose of vaccination, which lasts until the end of two consecutive hand, foot and mouth disease (HFMD) epidemic seasons (i.e., the peak incidence period; bimodal epidemics occurring within the same year shall be regarded as one single epidemic season).

The valid case monitoring period commences on Day 15 after the second dose, and the period prior thereto is defined as the surveillance window period. During case monitoring , suspected cases identified through active follow-up by investigators or spontaneous reports from participants' guardians shall be subjected to nucleic acid testing using pharyngeal swabs and/or anal swabs collected once within 3 days after the initial onset of symptoms, with subsequent follow-up and specimen sampling conducted according to test results.

Safety Assessment: Adverse events (AEs) occurring within 30 minutes after each vaccination shall be collected from all participants. Serious adverse events (SAEs) will be monitored from the first dose administration until 6 months after the last dose, with SAE follow-up visits conducted once monthly. All AEs identified via spontaneous reports from participants' guardians or any other sources from the first dose up to 30 days after the last dose will be closely monitored, truthfully recorded and included in the analysis.

A total of 3000 participants will be enrolled in the reactogenicity subgroup, including 1400 children aged 6-23 months and 1600 children aged 24-71 months. On the basis of the above safety observations, participants in the reactogenicity subgroup shall record solicited and unsolicited AEs within 0 to 7 days after each vaccination using diary cards, and collect AEs occurring from Day 8 to Day 30 after each vaccination via contact cards.

Humoral Immunogenicity Assessment: A total of 600 participants will be selected from the reactogenicity subgroup into the humoral immunogenicity subgroup, consisting of 280 children aged 6-23 months and 320 children aged 24-71 months. Venous blood samples of 3.0 (±0.5) mL will be collected from participants in this subgroup at baseline before the first dose, 30 days, 6 months and 12 months after the second dose respectively. Sera isolated at the above four time points will be tested for neutralizing antibodies against EV71, CA16, CA10 and CA6 to evaluate immunogenicity and immune persistence. Sera collected before the first dose and 30 days after the second dose will be used to detect neutralizing antibodies against different enterovirus subtypes so as to explore cross-protective capacity. Meanwhile, 60 participants aged less than 12 months from this subgroup will receive all vaccinations at the anterolateral thigh, while the remaining participants will be vaccinated via the deltoid muscle of the upper arm, to analyze the differences in immunogenicity and immune persistence between different vaccination sites.

Cellular Immunogenicity Assessment:Another 80 participants aged 24-71 months will be enrolled from the reactogenicity subgroup into the cellular immunogenicity subgroup. Venous blood samples of 3.0 (±0.5) mL will be collected before each vaccination and on Day 14 after each dose administration. Blood samples from the first 40 randomly numbered participants will be used to detect the expression levels of EV71 and CA16-specific IFN-γ and IL-4 in T cells, and samples from the latter 40 randomly numbered participants will be applied to measure T cell expression levels of CA10 and CA6-specific IFN-γ and IL-4.

The primary protective efficacy analysis will be conducted when case monitoring for at least one epidemic season is completed, the total number of hand, foot and mouth disease cases caused by any serotype of EV71, CA16, CA10 and CA6 identified during the valid case surveillance period reaches no less than 73, and the number of HFMD cases induced by CA16 and CA6 infection reaches no less than 38 respectively.

Inclusion Criteria:

1. Healthy children aged 6 to 71 months.
2. Guardians who can understand and voluntarily sign the informed consent form
3. Willing and able to comply with all visit schedules, sample collection, vaccinations, and other trial procedures。
4. Provide legal identification for the participant and their guardian

Exclusion Criteria:

1. A known history of HFMD/HA
2. Uncontrolled chronic diseases or a history of severe illnesses, including but not limited to cardiovascular diseases, blood disorders, liver or kidney diseases, digestive system diseases, respiratory system diseases, malignant tumors, or a history of major functional organ transplantation.
3. Autoimmune diseases, immunodeficiency diseases (including but not limited to systemic lupus erythematosus, ankylosing spondylitis, autoimmune thyroid diseases, asplenia, functional asplenia, and HIV infection).
4. Abnormal coagulation function (such as coagulation factor deficiencies,and platelet abnormalities).
5. Suffering from/having a history of severe neurological diseases (epilepsy, convulsions, or seizures [excluding a history of febrile seizures]) ,psychiatric disorders, or a family history of psychiatric disorders
6. Various acute diseases or exacerbations of chronic diseases within the last 3 days.
7. Having been vaccinated with a vaccine containing any of the components EV71, CA16, CA10, CA6.
8. Having received ≥14 days of immunosuppressive or other immunomodulatory treatment (prednisone ≥2mg/kg/day, or its equivalent; local or inhaled corticosteroids excluded) within the past 6 months, or cytotoxic treatment, or planning to receive such treatment during the trial.
9. Having received immunoglobulin or other blood products(excluding hepatitis B immunoglobulin) within the past 6 months, or planning to receive such treatment during the trial.
10. Having received other investigational drugs or vaccines within the past 30 days, or planning to receive such drugs or vaccines during the trial.
11. Having received live attenuated vaccines or nucleic acid vaccines within the past 14 days, or subunit or inactivated vaccines within the past 7 days.
12. Known allergy to any component of the investigational vaccine (inactivated EV71 virus, inactivated CA16 virus, , inactivated CA10 virus,, inactivated CA6 virus,aluminum hydroxide, sodium chloride, disodium hydrogen phosphate, sodium dihydrogen phosphate, injectable water).
13. On the day of planned vaccination with the investigational vaccine, having an axillary temperature ≥37.3°C before vaccination, or other vital sign measurements outside the normal range,or the physical examination is not qualified.
14. According to the investigator's judgment, participants have any other factors that make them unsuitable for participation in the clinical trial.