 This is a single-center, randomized study vaccine assigned to participants by chance, placebo-controlled study an inactive substance; a pretend treatment with no vaccine in it that is compared in a clinical trial with a vaccine to test if the vaccine has a real effect, double-blind neither the researchers nor the participants know what treatment the participant is receiving Phase 1 study in healthy participants. The study comprises a 4-week screening period; vaccination for each participant on Days 1, 85, and, 169; a 28-day follow-up period performed after each vaccination and a final visit at Day 337. Approximately 32 Participants will be randomized in parallel in a 3:3:2 ratio to 3 treatment groups Group 123 to receive either Ad26.RSV.FA2 or Ad35.RSV.FA2 or placebo. The study duration will be approximately 52 weeks. Blood samples for immunogenicity will be collected. Participants safety will be evaluated throughout the study. 