A Study to Evaluate the Safety and Immunogenicity of IVX-A12 in Participants of 60 to 85 Years of Age

The IVX-A12 Phase 2a clinical trial is a randomized, observer-blind, placebo-controlled, dosage optimization, multi-center trial to evaluate the safety and immunogenicity of a single intramuscular (IM) dose of IVX-A12, with or without adjuvant, in adults 60 to 85 years of age.

Participants will be administered a single shot of IVX-A12, at specified dosage levels, or placebo. The overall duration of the study is up to 1 year (12 months). A subset of participants will be followed for an additional 12 months for a total duration of 24 months.

Inclusion criteria

• Male or female participants, who must be in stable health based on medical history, vital signs, physical examination, and laboratory evaluation prior to vaccination, in the investigator's clinical judgment
• Participants may have ongoing chronic conditions (comorbidities such as hypertension, congestive heart failure, chronic obstructive pulmonary disease, type 2 diabetes mellitus, hyperlipoproteinemia, or hypothyroidism) who are, in the investigator's opinion, medically compensated and without recent exacerbation within the prior 3 months
• Participants able to voluntarily give written informed consent and can comply with trial procedures including follow-up for approximately 12 months after vaccination
• Body mass index 17 to less than (<) 40 kilograms per square meter (kg/m^2) at screening
• Before randomization, female participants must be unable to conceive (example, menopausal, that is, 12 consecutive months without menstruation, hysterectomy, oophorectomy, etc.) and not intending to conceive by any method
• Participants must agree not to donate blood from the time of vaccination through 3 months after vaccination
• Participants must be willing to provide verifiable identification and have the means to be contacted and to contact the investigator or the site's staff during the entire clinical trial

Exclusion criteria

• Participants with moderate or severe liver disease, metastatic solid tumor, and acquired immunodeficiency syndrome (AIDS) are to be excluded. In addition, participants with underlying significant illness or condition(s) or ongoing treatment that, in the opinion of the investigator, could (i) interfere with the conduct of the trial, (ii) pose an unacceptable risk to the participant in this trial, (iii) interfere with the participant's ability to comply with the trial procedures or abide by the procedures
• Older adults who meet frail elderly criteria (older persons with medical, nutritional, cognitive, emotional, or activity impairments, as defined by the Dalhousie Clinical Frailty Score greater than or equal to [>=]4)
• Prior receipt of any licensed or investigational RSV or hMPV vaccine within the past 12 months
• Prior receipt of another investigational medicinal product (IMP; trial drug, biologic, or device) not authorized for use in the United States of America (USA) and European Union within the past 3 months
• Laboratory-confirmed RSV or hMPV infection within 12 months prior to enrollment
• Currently enrolled or plan to participate in another clinical trial with an investigational agent (including licensed or unlicensed vaccine, drug, biologic, device, blood product, or medication) to be received during the trial period
• History of malignancy within 5 years before screening not in the following categories: (i) participants with squamous and basal cell carcinomas of the skin and carcinoma in situ of the cervix may be enrolled at the discretion of the investigator, (ii) participants with a history of malignancy within 5 years before screening, with minimal risk of recurrence per investigator's judgment, can be enrolled
• Acute illness, with or without fever at the time of planned vaccination
• History of hypersensitivity or serious adverse reactions to vaccines, such as anaphylaxis or angioedema, or any known allergies to any component of the IVX-121 and/or IVX-241 vaccine, or hypersensitivity to latex
• Abnormal function of the immune system resulting from clinical conditions including chronic administration of systemic corticosteroids (oral/intravenous/IM at a daily dose equivalent of greater than (>) 20 milligram (mg) prednisone in a period of more than 14 days), or administration of immunosuppressive chemotherapy, biologics, or radiotherapy within the past 3 months prior to planned vaccination
• Participants who have received treatment with immunoglobulins or other biologics, such as immunosuppressive therapies expected to modify immune response to vaccination (including monoclonal antibodies [MAbs] for chronic underlying conditions) within the past 3 months prior to planned vaccination
• Trial personnel as an immediate family or household member
• For licensed vaccines:

• Receipt of licensed inactivated vaccines (including seasonal influenza vaccine) within 14 days prior to trial vaccine administration on Day 0, or licensed replicating vaccines such as ribonucleic acid (RNA) or live-attenuated virus vaccines within 30 days prior to Day 0
• Receipt of licensed vaccines is permitted after completion of the Day 28 visit
• Receipt of any licensed Coronavirus Disease-2019 (COVID-19) vaccines is permitted if dosing regimen completed within 21 days prior to Day 0 or after completion of the Day 28 visit.