The purpose of this study is to evaluate the safety and immunogenicity of different formulations of vaccines encoding the RSV monovalent antigen or the Flu hemagglutinin subtype 5 (H5) antigen in healthy participants aged 18 to 49 years.

The total duration of study participation for each participant varies by stage and treatment arm.

Stage 1:

• For Arm 1, Arm 2, and Arm 3 the duration of study participation will be approximately 7 months for each participant.
• For Arm 4, Arm 5, and Arm 6 the duration of study participation will be approximately 6 months for each participant.

Stage 2: For all arms, the duration of study participation will be approximately 7 months for each participant.

Inclusion Criteria:

• Aged 18 to 49 years on the day of inclusion
• A female participant is eligible to participate if she is not pregnant or breastfeeding and of the following conditions applies:
• Is of NCBP (Non-Child-Bearing Potential). To be considered of NCBP, a female must be post-menopausal for at least 1 year, or surgically sterile. OR
• Is of CBP (Child-Bearing Potential) and uses an effective contraceptive method or abstinence from at least 4 weeks prior to study intervention administration until at least 12 weeks after the last study intervention administration.

Exclusion Criteria:

• Any condition which, in the opinion of the Investigator, might interfere with the evaluation of the study objectives

The above information is not intended to contain all considerations relevant to a patient's 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 design for Sentinel and Main Cohorts.

• Blinded to formulation: Participants; Investigators; Sponsor study staff (except biostatistician); Sponsor laboratory personnel (during interim analysis); Sponsor SMT for Main Cohorts only
• Unblinded to formulation: Staff preparing and administering study vaccines (at group level); Study biostatistician (throughout study); Sponsor SMT for Sentinel Cohorts, including ESDRs; SMT may receive unblinded data in case of identified safety concern evaluate the potential safety signal.
• Blinded to antigen assignment (RSV vs. Flu H5): None
• Unblinded to antigen assignment (RSV vs. Flu H5): Investigators; Participants; Sponsor study staff; Sponsor laboratory personnel (for immunogenicity assays); Study biostatistician; SMT