The purpose of this study is to evaluate the safety and immunogenicity of different liponanaoparticle (LNP) formulations of modified ribonucleic acid (mRNA) vaccines using the RSV pre-F antigen in healthy participants 18 to 49 years of age.

-Each participant will remain in the study for approximately 6 months.

• The study intervention will be administered as a single intramuscular (IM) dose in the upper arm on Day 1 (D01).
• The visit frequency will include 1 Screening Visit, 4 planned site visits to occur on D01, D03, D08, and D29, and 2 telephone calls to occur at Month (M)3 and M6.

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 agrees to use an effective contraceptive method or abstinence from at least 4 weeks prior to study intervention administration until at least 12 weeks after 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

Investigators, study site staff, laboratory personnel, outcome assessors, and participants will be blinded throughout the study. Study staff preparing and administering the study interventions will be unblinded throughout the study.