This clinical study will assess the safety, tolerability, and immunogenicity of VAL-181388 in healthy participants.

Inclusion Criteria:

• 18 to 49 years of age
• Body mass index between 18 and 35 kilograms (kg)/square meter (m^2)
• In good health as determined by medical history
• Female participants must be non pregnant and non lactating and meet one of the following criteria: a) post menopausal b) surgically sterile, or c) of childbearing potential and agree to use an adequate contraception method
• Male participants must use an acceptable method of birth control through 3 months after the final vaccination
• Agrees to comply with the study procedures and provides written informed consent
• Has access to a consistent and reliable means of telephone contact and agrees to stay in contact with the study site for the duration of the study

Exclusion Criteria:

• Any ongoing, symptomatic acute or chronic illness requiring medical or surgical care
• Female of childbearing potential and has a positive pregnancy test at screening or on the day of vaccination
• Abnormal vital signs or screening safety laboratory test results including liver enzyme tests
• Administration of an investigational product within 60 days, or 5 half-lives, whichever is longer
• Administration of any live attenuated vaccines within 4 weeks before enrollment or inactive vaccines within 2 weeks before enrollment, or plans to receive any vaccine during the active vaccination period
• Prior administration of a vaccine for chikungunya virus (CHIKV), dengue, Yellow Fever, tick-borne encephalitis, a history of confirmed or suspected CHIKV infection, or has lived in a CHIKV-endemic area greater than 1 year or cumulative stay of greater than 30 days in 5 years
• Prior administration of investigational agent using formulations similar to VAL-181388
• A history of hypersensitivity or serious reactions to previous vaccinations
• Any known or suspected autoimmune disease or immunosuppressive condition, acquired or congenital, as determined by medical history and/or physical examination
• A history of inflammatory arthritis
• Any neurologic disorder
• Prior administration of immunoglobulins and/or any blood products within the 3 months preceding the administration of the study drug or plans to receive such products at any time during the study
• Any chronic administration of an immunosuppressant or other immune modifying drug
• Daily or every other day administration of antipyretic or analgesic medication
• Any acute illness at the time of enrollment
• Any significant disorder of coagulation requiring ongoing or intermittent treatment
• A history of idiopathic urticaria
• A history of alcohol abuse or drug addiction
• A positive test result for drugs of abuse
• The participant has any abnormality or permanent body art (for example, tattoo) that, in the opinion of the investigator, would obstruct the ability to observe local reactions at the injection site
• Any condition that, in the opinion of the investigator, would pose a health risk to the participant if enrolled or could interfere with evaluation of the study drug or interpretation of study results
• A positive test result for hepatitis B surface antigen, hepatitis C virus antibody, or human immunodeficiency virus types 1 or 2 antibodies
• A history of active cancer (malignancy) in the last 10 years
• Donation of blood or blood products >450 milliliters (mL) within 30 days of dosing
• Is an employee or first degree relative of the Sponsor, clinical research organization (CRO), or study site personnel

The safety analysis set included all participants who received at least 1 dose of study drug (VAL-181388 or placebo).

Through 1 year following the last vaccination

The safety analysis set included all participants who received at least 1 dose of study drug (VAL-181388 or placebo).

Double Blind