A Study of Investigational RSV/hMPV Combination and Investigational hMPV Vaccines in Younger and Older Adults

Inclusion Criteria: Participants are eligible to be included in the study only if all of the following criteria apply: * Written informed consent obtained from the participant prior to performance of any study-specific procedure. * Participants who can and will comply with the requirements of the protocol (e.g., completion of the eDiary, return for follow-up visits, ability to access and utilize a phone or other electronic communications). * Note: For OA participants, in case of physical incapacity that would preclude the self-completion of the eDiaries, either site staff can assist the participant (for activities performed during site visits) and/or the participant may assign a caregiver to assist him/her with this activity (for activities performed at home). However, at no time will the site staff or caregiver evaluate the participant's health status while answering eDiaries or make decisions on behalf of the participant. * Body Mass Index (BMI) between 18 kg/m\^2 and 33 kg/m\^2, inclusive. Specific inclusion criteria for OA * A male or female between and including, 60 to 80 YOA at the time of the study intervention administration. * Healthy participants or medically stable patients as established by medical history, physical examination (and normal screening laboratory tests including Grade 1 laboratory abnormalities that are not-clinically significant in Phase 1 only). Specific inclusion criteria for YA * A male or female participant between and including 18 to 49 Y…