A phase II clinical trial of a 24-valent pneumococcal conjugate vaccine (PCV24) developed by Shanghai Reinovax Biologics Co., LTD will be conducted in Chinese adults aged 18 years and older. The study's objective is to evaluate the safety and immunogenicity of PCV24. The trial is a single-center, randomized, blinded, parallel-controlled, non-inferiority design II clinical trial.

Phase II clinical trial of the 24-valent pneumococcal conjugate vaccine (PCV24) developed by Shanghai Reinovax Biologics Co., LTD will be conducted in Chinese adults aged 18 years and older. The study's objective is to evaluate the safety and immunogenicity of PCV24. The active control vaccine is a 23-valent pneumococcal polysaccharide vaccine (23-valent pneumococcal polysaccharide vaccine, PPSV23) produced by the Chengdu Institute of Biological Products.

At least 992 participants will be enrolled, including 496 adults aged 18-60 and 496 elderly people aged ≥61. Participants will be randomized 1:1:1:1 to receive one dose of PCV24 formulation 1, PCV24 formulation 2, PCV24 formulation 3,or PPSV23.

Inclusion Criteria:

1. Volunteers aged 18 and above on the day of screening, and can provide legal identification;

2. Informed consent must be obtained from the volunteer and signed informed consent form;

3. Volunteers are able and willing to comply with the requirements of the clinical trial protocol and can attend all visits;

4. Armpit body temperature ≤ 37.0 °C on the day of enrollment.

   Exclusion Criteria:

5. Previous vaccination with pneumococcal conjugate vaccine, prior vaccination with pneumococcal polysaccharide vaccine for less than 3 years, or previous history of invasive disease caused by Streptococcus pneumoniae;

6. Have any history of severe allergies in the past, such as anaphylactic shock, allergic laryngeal edema, Henoch-Schonlein purpura, thrombocytopenic purpura, local allergic necrosis reaction (Athus reaction), etc.;

7. Have been diagnosed with congenital or acquired immunodeficiency, or have recently received immunosuppressant therapy, such as systemic glucocorticoid therapy such as prednisone or similar drugs >5 mg/day for more than 2 consecutive weeks in 1 month prior to vaccination);

8. Suffering from more serious cardiovascular diseases, such as arrhythmia, conduction block, myocardial infarction, severe hypertension that cannot be controlled by medication (systolic blood pressure ≥ 160 mmHg or diastolic blood pressure ≥100 mmHg when measured on site);

9. Those who have acute febrile illness or acute infectious disease or have cold symptoms, advanced influenza and take medication;

10. History of nervous system damage, severe congenital malformations, severe developmental disorders, severe genetic defects, severe malnutrition, etc.;

11. Have a history or family history of epilepsy, encephalopathy, or psychiatric disorders;

12. Those who have been diagnosed with thrombocytopenia, any abnormal coagulation function (such as coagulation factor deficiency, coagulation disorders, platelet abnormalities, etc.) or contraindications to intramuscular injection such as anticoagulant therapy;

13. asplenia, functional asplenia, and asplenia or splenectomy due to any cause;

14. Suffering from severe chronic diseases or the disease is in the advanced stage and cannot be controlled smoothly;

15. Have received blood or blood-related products or immunoglobulins within 3 months;

16. Received live attenuated vaccine within 14 days;

17. Other vaccinations within 7 days;

18. Received other investigational drugs or vaccines within 1 month;

19. Is participating in or plans to participate in clinical studies of other drugs or vaccines during the study period;

20. Any other condition that, in the opinion of the investigator, may interfere with the study evaluation;

21. Exclusion Criteria for Partial Populations:

Females of childbearing potential (18~49 years old): positive urine pregnancy test on the day of enrollment, lactation, or pregnancy plans within 6 months.