A Phase I clinical trial of 24-valent pneumococcal conjugate vaccine (PCV24) developed by Sinovac Life Science Co., Ltd will be conducted in adults aged 18 years and older. The objective of the study is to evaluate the safety and immunogenicity of Sinovac PCV24. The trial is a randomized, double-blind, controlled Phase I clinical trial.

A phase I clinical trial of the study of 24-valent Pneumococcal Polysaccharide Conjugate Vaccine (PCV24) developed by Sinovac Life Science Co., Ltd (Sinovac) will be conducted in Chinese adults aged 18 years and older. The trial is a randomized, double-blind and active controlled study. The objective of this study is to evaluate the safety and immunogenicity of PCV24 manufactured by Sinovac Life Science Co., Ltd. The active control vaccine is the Pneumovax® manufactured by MSD. The placebo is 0.9%NaCl solution.

A total of at least 168 participants will be enrolled, including 84 adults aged 18-59 years and 84 elderly people aged ≥60 years. Participants will be randomized in a 2:2:2:1 ratio to receive one dose of PCV24 formulation 1, PCV24 formulation 2, Pneumovax® or placebo.

Inclusion Criteria:

1. Healthy volunteers who are aged 18 years and older, and provide legal proof of identity;
2. Participants understand and voluntarily sign the informed consent form;
3. Participants can follow all study procedures and stay in contact during the study.

Exclusion Criteria:

1. Received any pneumococcal vaccine prior to enrollment;
2. History of invasive pneumococcal diseases (IPDs) or other pneumococcal diseases caused by Streptococcus pneumoniae, as confirmed by laboratory tests;
3. History of allergy or adverse reactions to the vaccine or vaccine components, or history of allergy, such as urticaria, dyspnea, angioedema and anaphylactic shock;
4. Congenital malformations or developmental disorders, genetic defects (such as Down syndrome, thalassemia, or G6PD deficiency), severe malnutrition;
5. Have uncontrolled chronic diseases or history of severe diseases, including but not limited to cardiovascular diseases, such as hypertension uncontrolled by drugs (measurement on site: systolic blood pressure ≥160 mmHg and/or diastolic blood pressure ≥100 mmHg) and coronary heart disease, metabolic diseases (such as uncontrolled diabetes), hematological diseases (e.g. severe anemia, hemophilia), liver and kidney diseases, digestive diseases, respiratory diseases (such as active tuberculosis), malignant tumors and major functional organ transplantation history;
6. Autoimmune diseases or immunodeficiency diseases (including but not limited to systemic lupus erythematosus, rheumatoid arthritis, autoimmune thyroid disease, asplenia, functional asplenia, HIV infection)
7. Diagnosed abnormal blood coagulation function (eg, lack of blood coagulation factors, blood coagulopathy, abnormal platelets level), history of obvious bleeding, hematoma or bruising after intramuscular injection or venipuncture.
8. Have/have suffered from a serious neurological disorder (epilepsy or convulsions) or mental illness or have a family history of such diseases.
9. Long-term alcohol or drug abuse.
10. Have received > 14 days of immunosuppressive or other immunomodulatory therapy (such as prednisone ≥20 mg/ day, or its equivalent) in the past 6 months, or cytotoxic therapy, or plan to receive such therapy during the study period.
11. Received immunoglobulin or other blood products within 3 months prior to enrollment, or plan to receive such treatment during the study period;
12. Received other investigational drugs or vaccines within 30 days prior to enrollment, or plan to receive such drugs or vaccines during the study;
13. Received live attenuated vaccine within 14 days prior to enrollment;
14. Received subunit or inactivated or other vaccine within 7 days prior to enrollment;
15. Acute diseases or acute onset of chronic diseases within 7 days prior to enrollment, or known or suspected active infection;
16. Women who are breastfeeding;
17. Fertile women aged 18-59 years who plan to become pregnant or cannot use effective contraception or have egg donation plans from enrollment to 3 months after the study intervention;
18. Women who are pregnant (as judged by the participant's blood pregnancy test);
19. Had fever (axillary temperature> 37.0℃) before vaccination;
20. Abnormalities in clinical laboratory indicators that exceed reference range and are clinically significant.
21. In the investigator's judgment, the participant has any other factors that make him or her unfit to participate in the clinical trial.