This study is a randomized, double-blind, active-controlled design. The goal is to evaluate the safety, pharmacokinetics and pharmacodynamic of SYN023 in combination with rabies vaccine in healthy participants under 18 years of age.

Participants will:

1. Be randomly assigned to receive one of two doses of SYN023 or a dose of HRIG by intramuscular injection on Day 0, along with the first dose of the rabies vaccine.
2. Receive additional doses of the rabies vaccine on Days 3, 7, 14, and 28.
3. Have all adverse events (within 42 days) and all serious adverse events (within 126 days) after PEP administration collected and recorded.
4. Provide several blood samples for pharmacokinetics and pharmacodynamic testing.

Inclusion Criteria:

1. Under 18 years of age at enrollment, male or female, with legal identification available.
2. Legal guardians of the volunteers voluntarily agree to participate in the study and sign the Informed Consent Form (ICF). Specifically, for volunteers under 8 years old, ICF is signed by legal guardians while fully respecting the child's opinion; for volunteers aged 8-17 years, legal guardians sign the ICF while volunteers themselves sign the ICF for minor volunteers;
3. Willing and able to comply with all study procedures, expected to be able to complete all follow-up visits and maintain contact throughout the study period;
4. Female volunteers of childbearing potential must have a negative urine pregnancy test prior to investigational product/vaccine administration, be non-lactating, and agree to use effective contraception during the study;
5. In good general health with normal physical examination findings, vital signs measurements, and axillary temperature ≤ 37.0℃.

Exclusion Criteria:

1. Subjects with history of injection of rabies vaccine and/or rabies virus passive immunization agents such as equine immunoglobulin, equine purified F (ab') 2 fragment products and HRIG;
2. Subjects with history of being bitten by a dog, cat, ferret, fox, ferret, skunk, bat or raccoon (wound with skin damage) in the past 6 months;
3. Subjects who have had pyrexia (≥ 37.3℃) or other acute illness within 7 days before enrollment, or are in acute exacerbation of chronic diseases;
4. History or current presence of any autoimmune or immunodeficiency disorders (including but not limited to systemic lupus erythematosus, ankylosing spondylitis, autoimmune thyroid disease, HIV infection, etc.);
5. Subjects with loss of splenic function or functional impairment, such as asplenia due to any condition (e.g., splenectomy);
6. Subjects with a history of severe allergy to previous vaccination requiring medical intervention, such as generalized urticaria, allergic laryngeal edema, Henoch-Schonlein purpura, local allergic necrosis reaction (Arthus reaction), angioneurotic edema and anaphylactic shock; or known hypersensitivity to any component contained in the investigational products/vaccine;
7. History or current presence of any systemic disease or poorly controlled chronic condition that may interfere with safety or efficacy evaluations as determined by the investigator, including but not limited to hematologic disorders, hepatic/renal diseases, gastrointestinal disorders, respiratory diseases, malignancies, or history of major organ transplantation, etc.;
8. History or current presence of severe neurological disorders (e.g., epilepsy, convulsions or seizures [excluding febrile seizures], encephalopathy) or psychiatric illness, or family history of psychiatric disorders;
9. Presence of coagulation abnormalities (e.g., coagulation factor deficiency, platelet disorders);
10. Administration of immunoglobulins or blood products within 3 months prior to enrollment, or planned use during the study;
11. Receipt of systemic immunosuppressants or other immunomodulatory therapies within 3 months prior to enrollment, including but not limited to systemic corticosteroids (e.g., prednisone ≥ 2 mg/kg/day for > 2 weeks), or cytotoxic therapy, or planned administration during the trial;
12. Participation in other clinical trials within 3 months prior to enrollment, or planned participation during the study;
13. Administration of live-attenuated vaccines within 14 days or subunit/inactivated vaccines within 7 days prior to enrollment;
14. Presence of skin lesions, inflammation, ulcers, rashes, or scars at the intended injection site that may interfere with administration or local reaction assessment;
15. Any other condition considered by the investigator to render the participant unsuitable for study participation.