This study is a Phase 3, randomized, modified double-blind study which aims to measure whether the investigational pneumococcal conjugate vaccine PCV21 is safe and can help the body to develop germ-fighting agents called "antibodies" (immunogenicity) when it is given after 1 dose, 2 doses, or 3 doses of a licensed 20-valent pneumococcal vaccine compared to when 20-valent pneumococcal vaccine is given as a complete series in infants aged from approximately 2 months (42 to 89 days).

The study duration per participant will be up to approximately 19 months. The study vaccines (either PCV21 or 20vPCV) will be administered at approximately 2, 4, 6 and 12 to 15 months of age (MoA). Routine pediatric vaccines will be given as per local recommendations.

There will be 6 study visits:

Visit (V)01, V02 separated from V01 by 60 days, V03 separated from V02 by 60 days, V04 separated from V03 by 30 days, V05 at 12 months of age until 15 months of age, V06 separated from V05 by 30 days.

Inclusion Criteria:

• Aged 42 to 89 days on the day of inclusion
• Participants who are healthy as determined by medical evaluation including medical history and physical examination
• Born at full term of pregnancy (≥ 37 weeks) and with a birth weight ≥ 2.5 kg or born after a gestation period above 28 (> 28 weeks) through 36 weeks with a birth weight ≥ 1.5 kg, and in both cases medically stable as assessed by the investigator

Exclusion Criteria:

Participants are excluded from the study if any of the following criteria apply:

• Known or suspected congenital or acquired immunodeficiency; or receipt of immunosuppressive therapy, such as anti-cancer chemotherapy or radiation therapy; or long-term systemic corticosteroid therapy
• History of microbiologically confirmed Streptococcus pneumoniae infection or disease.
• History of seizure or significant stable or progressive neurologic disorders such as inflammatory nervous system diseases, encephalopathy, cerebral palsy
• Known systemic hypersensitivity to any of the study interventions components, or history of a life-threatening reaction to the study interventions used in the study or to a product containing any of the same substances
• Thrombocytopenia, or known thrombocytopenia, as reported by the parent/legal acceptable representative (LAR), contraindicating intramuscular (IM) injection.
• Bleeding disorder, or receipt of anticoagulants in the 3 weeks preceding inclusion, contraindicating IM injection
• Chronic illness that, in the opinion of the investigator, is at a stage where it might interfere with study conduct or completion
• Moderate or severe acute illness/infection (according to investigator judgment) or febrile illness (temperature ≥ 38.0°C [≥ 100.4°F]) on the day of study intervention administration. A prospective participant should not be included in the study until the condition has resolved or the febrile event has subsided.
• Previous vaccination against Streptococcus pneumoniae
• Receipt of immune globulins, blood or blood-derived products since birth
• Participation at the time of study enrollment (or in the 6 weeks preceding the first study intervention administration) or planned participation during the present study period in another clinical study investigating a vaccine, drug, medical device, or medical procedure

Note: The above information is not intended to contain all considerations relevant to a potential participation in a clinical trial.

Qualified researchers may request access to patient level data and related study documents including the clinical study report, study protocol with any amendments, blank case report form, statistical analysis plan, and dataset specifications. Patient level data will be anonymized and study documents will be redacted to protect the privacy of trial participants. Further details on Sanofi's data sharing criteria, eligible studies, and process for requesting access can be found at: https://vivli.org

Modified double-blind

• Blinding for vaccine group assignment: participants and participant's parent(s) / legally acceptable representative(s) (LARs), outcome assessors, Investigators, laboratory personnel, and Sponsor study staff
• No blinding for vaccine group assignment: those preparing and administering the study interventions