This study is a randomized, double-blind, placebo-controlled Phase I clinical trial representing the first-in-human (FIH) investigation of the Monkeypox recombinant protein vaccine JT118. The primary objectives are to evaluate the safety, tolerability, and immunogenicity of two doses of JT118 in healthy adults and to conduct preliminary exploration of vaccination regimens.

The study will sequentially proceed from the low-dose group to the high-dose group with Subgroup 1 (participants without prior Smallpox vaccination history) and Subgroup 2 (participants with prior Smallpox vaccination history) being conducted concurrently within each dose group. It will explore single-dose, two-dose, and three-dose vaccination regimens with approximately 180 healthy adult participants planned for enrollment.

Inclusion Criteria:

1. Male or female participants aged 18 to 59 years (inclusive) at the time of signing the informed consent form.
2. Participants with fertility (including male participants) and their spouses are willing to avoid pregnancy (including sperm donation and egg donation plans) from 14 days before screening until six months after the last administration of the investigational vaccine, and are willing to use effective contraception.
3. Ability to understand the study procedures, sign a written informed consent form, and complete the study according to the protocol requirements.
4. Body temperature <37.3°C (axillary temperature) on the day of enrollment.

Exclusion Criteria:

1. Individuals with a history of Smallpox infection or Monkeypox infection or close contact with individuals infected with Monkeypox.
2. Individuals whose medical history or clinical evidence indicates presence of significant comorbidities (including but not limited to cardiovascular system disorders; respiratory system disorders; digestive system disorders; urinary system disorders; neurological disorders; hematological disorders; immunological disorders; endocrine/metabolic disorders; infectious diseases; psychiatric/psychological disorders) where the investigator determines them unsuitable for enrollment.
3. Individuals whose pre-vaccination baseline assessments (including vital signs examination/physical examination/laboratory test/ECG) show clinically significant abnormalities where the investigator determines them unsuitable for enrollment.
4. Individuals testing positive for human immunodeficiency virus antibody/HBV surface antigen (HBsAg)/HCV virus antibody/Syphilis spirochete antibody .
5. Individuals who have previously received an approved or investigational Monkeypox vaccine.
6. Use of any immunosuppressive drugs within 28 days before the first dose.
7. Receipt of an inactivated vaccine within 14 days before the first dose or receipt of a live attenuated vaccine within 30 days before the first dose.

8 .Major surgery received within six months before receiving the first dose or anticipated major surgery during study participation.

9. Blood donation history (excluding physiological female bleeding) or significant haemorrhage (≥200 mL) within three months prior to receiving the first dose; or planned blood donation (including component blood donation) within one hundred eighty days after last vaccination.

10. Use of immunoglobulins and/or any blood products within three months prior to receiving initial vaccination; or planned use during trial participation.

11 .Participants who have used any investigational or unregistered products (including drugs, vaccines, biologics, or devices) within 3 months prior to receiving the first dose of study medication, or plan to use such products during the study period.

12. Participants with known allergy to any excipient(s) of this product. 13 .Participants with extensive scars or skin disorders at potential injection sites.

14 .Women who are pregnant or lactating/breastfeeding. 15 .Participants with a history of drug abuse within 12 months prior to screening OR positive drug screen results before randomization.

16. History of alcohol dependence within 6 months prior to screening, or positive alcohol breath test result before randomization.

17. Participants deemed unsuitable for participation by investigators due to other reasons (e.g., presence of concomitant medical conditions/history, poor compliance).

18 .Participants who experienced a severe allergic reaction after administration of the first or second dose* of vaccination.

19. Participants who experienced a serious adverse event (SAE) likely or definitely related to investigational vaccine following administration of first/second dose*, with decision on continued participation made by investigators.

20. Participants who failed initial dose eligibility criteria (inclusion/exclusion) following administration of first/second dose*, with decision on continued participation made by investigators .

21 .Presence of any other medical condition considered unsuitable for vaccination by investigators.

This study was a randomized double-blind trial where participants, investigators, and sponsors were unaware of participants' treatment assignments. non-Blind research staff were responsible for medication preparation