This is a Phase 1, FIH, single-center, randomized, placebo controlled, dose escalation study to evaluate the safety, tolerability, PK, and PD of mRNA-1944 in healthy adult subjects. Cohorts of mRNA-1944 are planned to be investigated in a sequential dose escalation manner.

Inclusion Criteria:

• Male or female ≥ 18 and ≤ 50 years of age
• Weight of 50 to 100 kg, inclusive
• In good general health as determined by medical history, clinical laboratory assessments, ECG results, vital sign measurements, and physical examination findings at screening
• Has access to consistent and reliable means of contact and agrees to stay in contact with the study site for the duration of the study

Exclusion Criteria:

• Any acute or chronic clinically significant disease, as determined by physical examination or laboratory screening tests
• Elevated liver function tests or safety laboratory test results
• Positive screening test for the presence of anti-CHIKV IgG
• Administration of another investigational study involving any investigational product within 60 days or 5 half-lives, whichever is longer
• Has received any live attenuated or inactive vaccines within 4 weeks prior to check-in, or plans to receive any vaccine during the study
• Known or suspected immune-mediated disease or immunosuppressive condition (including lymphoproliferative disorders)
• Any neurologic disorder
• History of idiopathic urticaria
• Any bleeding disorder that is considered a contraindication to study drug infusion or blood collection
• Receipt of immunoglobulins, a monoclonal antibody or any blood products within the preceding 6 months
• Any acute illness at the time of enrollment
• A positive test result for drugs of abuse
• A positive test result for hepatitis B surface antigen, hepatitis C virus antibody, or human immunodeficiency virus type 1 or 2 antibodies
• A history of active cancer (malignancy) in the last 3 years
• Donation of ≥ 450 mL blood or blood products within 30 days of study drug infusion