Evaluating a new vaccine for Hand, Foot, and Mouth Disease in young children

Inclusion Criteria:

* Healthy children aged 6 to 71 months with no history of EV71 vaccination; or healthy children aged 24 to 71 months who have completed two doses of EV71 vaccine at least 6 months prior.
* Guardians who can understand and voluntarily sign the informed consent form.
* Willing and able to comply with all visit schedules, sample collections, vaccinations, and other research procedures.
* Provide proof of identity documents.

Exclusion Criteria:

* A known history of HFMD/HA.
* 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.
* Autoimmune diseases, immunodeficiency diseases (including but not limited to systemic lupus erythematosus, ankylosing spondylitis, autoimmune thyroid diseases, asplenia, functional asplenia, and HIV infection).
* Abnormal coagulation function (such as coagulation factor deficiencies, and platelet abnormalities).
* 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.
* Various acute diseases or exacerbations of chronic diseases within the last 3 days, or known or suspected active infections.
* Received a vaccine containing CA16 components.
* Received immunosuppressive or other immunomodulatory treatments for ≥14 days within the past 6 months (prednisone ≥2mg/kg/day, or its equivalent; local or inhaled corticosteroids excluded), cytotoxic therapy, or planning to receive such treatment during the trial.
* Received immunoglobulin or other blood products within the past 6 months, or planning to receive such treatment during the trial.
* Received other investigational drugs or vaccines within the past 30 days, or planning to receive such drugs or vaccines during the trial.
* Received live attenuated vaccines or nucleic acid vaccines within the past 14 days, or subunit or inactivated vaccines within the past 7 days.
* Known allergy to any component of the investigational vaccine (inactivated EV71 virus, inactivated CA16 virus, aluminum hydroxide, sodium chloride, disodium hydrogen phosphate, sodium dihydrogen phosphate, injectable water).
* On the day of the planned vaccination with the trial vaccine, there is a fever, with axillary temperature \> 37.0°C before vaccination;
* On the day of the planned vaccination with the trial vaccine, the physical examination is not qualified.
* Skin damage, inflammation, ulcers, rash, or scars at the target injection site that may interfere with vaccination or observation of local reactions.
* According to the investigator's judgment, participants have any other factors that make them unsuitable for participation in the clinical trial.