This is a phase 3 randomized, modified double-blind study whose purpose is to measure whether 3 lots of the investigational pneumococcal vaccine PCV21 can help the body to develop germ-fighting agents called "antibodies" (immunogenicity) in a similar way (ie, same immune response) when they are given to infants aged from approximately 2 months (42 to 89 days) and are safe compared to a licensed 20-valent pneumococcal vaccine (20vPCV) (Prevnar 20™).

The study duration per participant will be up to approximately 17 months. The study vaccines (either PCV21 or 20vPCV) will be administered at approximately 2, 4, 6 and 12 months of age. Cohort A will include randomization to three PCV21 formulation groups or one 20vPCV comparator group (Group 1-4, approximately 896 total participants), whereas Cohort B will include randomization to three PCV21 formulation groups only (Groups 1-3, approximately 1299 total participants). 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, 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

Exclusion Criteria:

• 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
• Any contraindication to the routine pediatric vaccine being administered in the study
• History of seizure or significant stable or progressive neurologic disorders such as infantile spasms, 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 substances3
• Laboratory-confirmed or known thrombocytopenia, as reported by the parent(s) / legally acceptable representative (LAR(s)), 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.
• Receipt of any BCG vaccine within 4 weeks preceding the first study intervention administration or planned receipt any BCG vaccine within the study period
• Previous vaccination against Streptococcus pneumoniae
• Previous vaccination against the following antigens: diphtheria, tetanus, pertussis, H. influenzae type b, poliovirus
• Receipt of more than 1 dose of hepatitis B vaccine
• 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

Participants, participant's parent(s) / LAR(s), Sponsor study staff, investigators and study staff, will remain blinded during the study conduct, except dedicated study staff preparing/administering the study interventions who will be blinded regarding the PCV21 lot but unblinded regarding the participants, randomized in the 20vPCV arm. The unblinded study staff will not be involved in any safety evaluation.