In the treatment of pneumococcal diseases, the common use of penicillin-based antimicrobial agents has led to drug resistance, which has become a global challenge. Therefore, disease prevention through vaccination is essential. The 23-valent pneumococcal polysaccharide vaccine, the first widely used vaccine, has limitations. Subsequent pneumococcal polysaccharide conjugate vaccines have improved protection rates but increased cases of infections caused by non-vaccine serotype strains. Currently, vaccines available in China for pneumococcal disease prevention include the 23-valent polysaccharide vaccine (approved only for adults) and the 13-valent conjugate vaccine. PCV24 expands serotype coverage and converts capsular polysaccharides into T-cell-dependent antigens by binding them to proteins, stimulating humoral immunity and the complement system to generate specific antibodies and immunological memory for disease prevention. This study aims to preliminarily investigate the safety and immunogenicity of PCV24 vaccination in Chinese adults.

Streptococcus pneumoniae is a Gram-positive diplococcus, and its capsular polysaccharide is a key virulence factor. According to differences in the composition and structure of capsular polysaccharides, it can be divided into more than 90 serotypes, with varying viability and pathogenicity among different serotypes. As an important opportunistic pathogen, this bacterium can breach mucosal defense systems to cause invasive infections when host immunity declines, leading to diseases such as pneumonia, bacterial meningitis, bacteremia, sinusitis, and otitis media.

Antimicrobial agents commonly used in the treatment of pneumococcal diseases, such as penicillins, macrolides, and cephalosporins, have faced global challenges due to drug resistance, making vaccination essential for disease prevention. The World Health Organization (WHO) has classified pneumococcal diseases as a top priority for vaccination. The early widely used 23-valent pneumococcal polysaccharide vaccine had limitations, while subsequent 7-valent, 10-valent, and 13-valent pneumococcal polysaccharide conjugate vaccines improved protection rates but were associated with increased infections from non-vaccine serotype strains. Currently, only the 23-valent polysaccharide vaccine is approved for adult use in China. Pneumococcal conjugate vaccines convert capsular polysaccharides into T-cell-dependent antigens by binding them to proteins, thereby stimulating humoral immunity and the complement system to generate specific antibodies and immunological memory for disease prevention. This study aims to evaluate the safety and tolerability of a 24-valent pneumococcal polysaccharide conjugate vaccine in individuals aged 18 years and older.

Inclusion Criteria:

• 1.Age and Identification: Volunteers aged ≥18 years on the screening day, able to provide valid legal identification.

  2.Informed Consent: Volunteers who have received and understood the study information, voluntarily agreed to participate, and signed the Informed Consent Form.

  3.Compliance: Volunteers able and willing to adhere to the clinical trial protocol requirements and attend all scheduled visits.

Baseline Temperature: Axillary temperature ≤37.0°C on the enrollment day.

Exclusion Criteria:

• 5.Vaccination History: Prior receipt of pneumococcal conjugate vaccine; pneumococcal polysaccharide vaccine within the past 3 years; or a history of invasive Streptococcus pneumoniae disease.

  6.Severe Allergic Reactions: History of severe allergic reactions, such as anaphylactic shock, allergic laryngeal edema, allergic purpura, thrombocytopenic purpura, local allergic necrotic reaction (Arthus reaction), etc.

  7.Immunocompromised Status: Diagnosed congenital or acquired immunodeficiency; or receipt of systemic glucocorticoid therapy (e.g., prednisone or equivalent >5 mg/day for ≥2 consecutive weeks) within 1 month prior to vaccination (local, inhaled, or nebulized steroids are permitted).

  8.Severe Cardiovascular Diseases: History of arrhythmia, conduction block, myocardial infarction, or uncontrolled severe hypertension (on-site measurement: systolic blood pressure ≥160 mmHg and/or diastolic blood pressure ≥100 mmHg).

  9.Acute Illness or Medication Use: Acute febrile illness, acute infectious disease, active cold symptoms, progressive influenza, or current use of related therapeutic medications.

  10.Neurological/Developmental Disorders: History of neurological impairment, severe congenital malformation, severe developmental disorder, severe genetic defect, or severe malnutrition.

  11.Neuropsychiatric History: Personal or family history of epilepsy, encephalopathy, or psychiatric disorders.

  12.Coagulation Abnormalities: Diagnosed thrombocytopenia, coagulation dysfunction (e.g., coagulation factor deficiency, coagulation disorders, platelet abnormalities), or contraindications to intramuscular injection (e.g., anticoagulant therapy).

  13.Splenic Abnormalities: Asplenia, functional asplenia, or splenectomy for any reason.

  14.Uncontrolled Chronic Diseases: Severe chronic diseases or progressive conditions that cannot be stably controlled.

  15.Blood Products/Immunoglobulins: Receipt of blood or blood products, or immunoglobulins within the past 3 months.

  16.Live Attenuated Vaccines: Vaccination with live attenuated vaccines within the past 14 days.

  17.Other Vaccines: Vaccination with other vaccines within the past 7 days. 18.Investigational Products: Receipt of other investigational drugs or vaccines within the past 1 month.

  19.Concurrent Studies: Current participation or planned participation in another clinical study of drugs or vaccines during the research period.

  20.Investigator's Discretion: Any other condition that, in the investigator's judgment, may interfere with study evaluation.

  21.Additional Exclusions for Specific Populations: Females of Childbearing Potential (18-49 years): Positive urine pregnancy test on enrollment day; lactation; or planning to become pregnant within 6 months.