This trial employs a randomized, double-blind, placebo/positive control, and dose-finding design to evaluate the safety, tolerability, pharmacokinetics, pharmacodynamics, immunogenicity, and preliminary efficacy of SIBP-A16 injection in premature and term infants.

This study established three study groups: the test drug group, the placebo group, and the positive control group. Four dose cohorts were set up: Cohort 1 (Dose 1), Cohort 2 (Dose 2), Cohort 3 (Dose 3), and Cohort 4 (Dose 2). A total of 36 participants were enrolled. The drug will be administered via intramuscular injection as a single dose. Initially, Cohort 1 enrolled 7 participants, who were randomly assigned to receive either one dose of the test drug or placebo. After completing the initial 14-day safety observation, if the dose escalation termination criteria were not triggered, participants were enrolled into Cohort 2 (11 participants, randomly assigned to receive either one dose of the test drug or placebo). Once Cohort 2 was fully enrolled, participants could be enrolled into Cohort 4 (7 participants, randomly assigned to receive either one dose of the positive control drug or placebo). After Cohort 2 completed the 14-day safety observation, participants were enrolled into Cohort 3 (11 participants, randomly assigned to receive either one dose of the test drug or placebo) following the same procedure. If the dose escalation termination criteria were triggered, the Data Monitoring Committee (DMC) would conduct a safety assessment and discuss with the research team and sponsor whether to terminate the dose escalation.

Inclusion Criteria:

• During screening, infants within 1 year of age, including premature infants (gestational age ≥29 to <35 weeks) and full-term infants (gestational age ≥35 weeks), with underlying diseases but no other risk factors, are allowed to participate in the trial;
• Infants with a body weight ≥3 kg at screening;
• Infants who are entering their first RSV infection season at screening;
• Parents/legal guardians of trial participants have signed the informed consent form;
• Parents/legal guardians of trial participants are able to understand and comply with the requirements and procedures of the protocol, including scheduled center visits, telephone interviews, and blood sample collection;
• Participants can complete the follow-up period, which is approximately 1 year after the administration of the study drug.

Exclusion Criteria:

• Any fever (≥37.5°C, axillary temperature) or acute illness (defined as the presence of moderate or severe symptoms or signs) occurring within 7 days prior to drug administration;
• Having experienced Lower Respiratory Tract Infections (LRTI) within the previous 7 days prior to randomization, or having active LRTI at the time of randomization;
• Individuals with chronic eczema or urticaria, or those with an allergic constitution who are allergic to multiple drugs, or those with a known history of allergy to immunoglobulin products, blood products, other exogenous proteins, or any components of this product;
• Had a history of RSV infection before randomization, or had active RSV infection at the time of randomization;
• Those who have received non-oral inactivated vaccines or component vaccines within 7 days before administration;
• Having received a non-oral live attenuated vaccine within 30 days prior to drug administration;
• Participants who have received any medication within 7 days prior to drug administration, except for: a) various vitamins and iron supplements; b) systemic over-the-counter medications (such as analgesics) for common pediatric symptoms, which may be used occasionally, as determined by the investigator;
• Participants with autoimmune diseases who are currently receiving, or are expected to receive according to the investigator's judgment, immunosuppressive therapy (including steroids, excluding topical steroids) during the trial period;
• Have previously used or are expected to receive blood products or immunoglobulin products during the trial period;
• Known renal dysfunction or liver dysfunction;
• Known to have chronic lung disease (CLD)/bronchopulmonary dysplasia;
• Congenital respiratory abnormalities with clinical significance;
• Suffering from congenital heart disease (CHD) accompanied by significant hemodynamic changes;
• Suffering from chronic epilepsy or progressive or unstable neurological disorders;
• Those who have previously experienced or are suspected to have experienced life-threatening acute events, and are still deemed unsuitable for participating in clinical trials by the researchers;
• Known immune deficiency, including infection with human immunodeficiency virus (HIV);
• The mother is infected with HIV (unless it has been proven that the trial participant is not infected);
• The mother received the RSV vaccine during pregnancy;
• Have received any investigational drugs or participated in any intervention studies;
• Any other circumstances that the researcher believes may interfere with the evaluation of the study drug or the interpretation of the study results;
• The participants are the children of the researchers, their subordinate researchers, relatives, or staff members of the sponsor.