The main purpose of this study is to evaluate the safety and immunogenicity of three dosage levels (low, medium, high) of the bivalent combination respiratory syncytial virus (RSV)/human metapneumovirus (hMPV) virus-like particle (VLP) candidate vaccine (IVX-A12), compared to placebo, when administered as a single-dose regimen in healthy older adults 60 to 75 years of age.

The Phase 1 clinical trial of IVX-A12 is a randomized, observer-blinded, placebo-controlled, multi-center study designed to evaluate the safety and immunogenicity of multiple dosage levels of IVX-A12, with and without CSL Seqirus' proprietary adjuvant MF59®.

A total of up to 120 healthy older adults aged 60 to 75 years. Participants will be administered a single shot of IVX-A12, at one of three combination dosage levels below, or placebo. The overall duration of the study is up to 1 year (365 days).

Inclusion Criteria:

• Healthy male or non-pregnant female older adults 60 to 75 years of age at the time of first vaccination
• Participants with stable well-controlled chronic conditions such as hypertension without clinical exacerbation of their underlying disease within the previous 12 months
• Participants able to voluntarily give written informed consent and to comply with study procedures including follow-up to approximately 12 months after first dosing
• Body mass index (BMI) 17 to 35 kilogram per square meter (kg/m^2), inclusive, at screening
• Screening laboratory values must be within the laboratory reference ranges or deemed not clinically significant if within Grade 1 severity on the toxicity scale

Exclusion Criteria:

• Prior receipt of any investigational RSV or hMPV vaccine

• Prior receipt of another investigational medicinal product (study drug, biologic, or device) not authorized for use in the United States and European Union within the past year

• Laboratory-confirmed severe RSV or hMPV infection within the past year prior to enrollment

• Currently enrolled or plan to participate in another clinical study with an investigational agent (including licensed or unlicensed vaccine, drug, biologic, device, blood product, or medication) to be received during the study period

• Presence of high-risk comorbidities for severe RSV or hMPV disease (example, significant cardiopulmonary disease)

• Older adults meeting frail elderly criteria (older persons with medical, nutritional, cognitive, emotional, or activity impairments, as defined by the study site)

• Acute or chronic progressive, unstable or uncontrolled clinical conditions

• Acute illness, with or without fever at the time of planned vaccination

• History of hypersensitivity or serious adverse reactions to vaccines, such as anaphylaxis, Guillain-Barré, and 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 human immunodeficiency virus, chronic administration of systemic corticosteroids (oral/intravenous/IM at a dose equivalent of greater than (>) 20 milligrams (mg) prednisone in a period of more than 14 days), or administration of immunosuppressive chemotherapy, biologics, or radiotherapy within the past 3 months before study randomization

• Refusal to maintain contraceptive practices during the study, and (for women of childbearing potential) to be screened for pregnancy at specified times during the study

• Receipt of licensed inactivated vaccines including influenza vaccine within 14 days prior to study vaccine administration on Study Day 0, or with live virus vaccines within 30 days of Day 0

  1. Receipt of licensed vaccines is permitted after completion of the study Day 28 visit.
  2. Receipt of licensed COVID-19 vaccines is permitted if dosing regimen completed within 21 days prior to study vaccine administration on Day 0 or after completion of the Day 28 visit.