The primary purpose of this study is to assess the immunogenicity and safety of IVX-A12 in adults 60 years of age and older.

This is a randomized, modified double-blind, active controlled study to characterize the immunogenicity and safety of IVX-A12.

Approximately 140 participants will be randomized in a 1:1 ratio to receive IVX-A12 (approximately 70) or licensed respiratory syncytial virus (RSV) vaccine (AREXVY) (approximately 70).

The study is planned to be conducted at approximately 5 sites in the United States. The duration of each participant's involvement in the study will be approximately 6 months following administration of study vaccination.

Inclusion Criteria:

1. Adults >=60 years of age at the time of signing informed consent.
2. Participants who are medically stable according to the judgment of the Investigator.
3. Able to understand and comply with study requirements/procedures based on the assessment of the Investigator.
4. Capable of giving signed informed consent.

Exclusion Criteria:

1. Acute (time-limited) or febrile (temperature >=38.0 °C [100.4 ºF]) illness/infection within 3 days of planned dosing.
2. History of a clinically significant bleeding disorder (e.g., factor deficiency, coagulopathy, or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venipuncture.
3. History of hypersensitivity to any component of the study vaccination.
4. History of severe adverse reaction associated with a vaccine and/or severe allergic reaction (e.g., anaphylaxis).
5. Known or suspected congenital or acquired immunodeficiency.
6. Known or suspected autoimmune condition as determined by history and/or physical examination.
7. History of Guillain-Barré syndrome or any other demyelinating condition.
8. History of malignancy other than treated non-melanoma skin cancers or locally-treated cervical cancer in previous 5 years.
9. Any condition that may significantly increase the risk to the participant because of participation in the study, impact the participant's ability to participate in the study, or impair the interpretation of the study data.
10. Receipt of any licensed or investigational RSV and/or Human metapneumovirus (hMPV) vaccine any time prior to administration of study intervention.
11. Receipt of any licensed vaccine (other than licensed influenza or COVID-19 vaccines) within 28 days prior to or expected receipt within 28 days after administration of study intervention. Licensed influenza or COVID-19 vaccines are permitted beginning greater than (>)14 days prior to and >14 days after administration of study intervention.
12. Receipt of immunoglobulin or blood products within 3 months prior to administration of study intervention or expected receipt during the study.
13. Receipt of immune-modifying drugs or immunosuppressive therapy, such as anti-cancer chemotherapy or radiation therapy within 6 months prior to enrollment (or expected receipt during study), or long-term systemic corticosteroid therapy (prednisolone or equivalent at a dose of >=20 mg daily or every other day for more than 2 consecutive weeks) within 6 months prior to study intervention or anticipated receipt during study.
14. Participation in another study or receiving interventional study investigational medicinal product (IMP), in the preceding 28 days or expected receipt of another study intervention (or participation in another study) during the period of study follow-up.
15. Employees of the Sponsor involved in planning, executing, supervising, or reviewing the IVX-A12 program, clinical study site staff, or any other individuals involved with the conduct of the study, or immediate family members of such individuals.
16. Alcohol or substance abuse that, in the opinion of the Investigator, might interfere with the study conduct or completion.
17. Deprived of freedom by an administrative or court order, or in emergency setting, or hospitalized involuntarily.
18. Judgment by the Investigator that the participant should not participate in the study if the participant is unlikely to comply with study procedures, restrictions, and requirements.

The Safety analysis set (SAF) included all randomized participants who received one dose of study intervention, irrespective of their protocol adherence and continued participation in the study.

Day 1 through Day 181