The purpose of this study is to assess the safety and tolerability of intramuscular prime-boost regimens of Ad26.RSV.FA2 given either once or twice followed by Ad35.RSV.FA2 (human adenovirus-vectored vaccine candidate) in healthy participants.

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 1/2/3) 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. Participant's safety will be evaluated throughout the study.

Inclusion Criteria:

• Participant must be in good health, without significant medical illness, on the basis of physical examination, medical history, vital signs measurement, and 12-lead electrocardiogram (ECG) performed at screening
• Participant must meet protocol defined laboratory criteria within 28 days before Day 1
• Before randomization, a woman must be either; Not of childbearing potential: postmenopausal or surgically sterilized; of childbearing potential and practicing an effective method of birth control before vaccination and through 3 months after the last vaccination. Women, who are not heterosexually active at screening, must agree to utilize highly-effective method of birth control if they become heterosexually active until 3 months after receiving the last dose of study vaccine
• A woman must have a negative serum pregnancy test (beta-human chorionic gonadotropin [beta-hCG]) at the screening visit, and a negative urine pregnancy test pre-vaccination on Day 1
• A woman must agree not to donate eggs (ova, oocytes) for the purposes of assisted reproduction until 3 months after receiving the last dose of study vaccine. A man must agree not to donate sperm until 3 months after receiving the last dose of study vaccine

Exclusion Criteria:

• Participant has a body mass index (BMI) less than or equal to (<=)19 and greater than or equal to (>=30) kilogram per square meter (kg/m2)
• Participant has any clinically significant acute or chronic medical condition that, in the opinion of the investigator, would preclude participation (e.g. history of seizure disorders, bleeding/clotting disorder, autoimmune disease, active malignancy, poorly controlled asthma, active tuberculosis or other systemic infections)
• Participant has had major surgery within the 4 weeks prior to randomization or has planned major surgery through the course of the study
• Participant has chronic active hepatitis B or hepatitis C infection, documented by hepatitis B surface antigen and hepatitis C antibody, respectively
• Participant has human immunodeficiency virus (HIV) type 1 or type 2 infection