Safety and Immunogenicity of CHIKV VLP Vaccine PXVX0317 in Adults ≥65 Years

The purpose of this phase 3, randomized, double-blind, placebo-controlled study is to evaluate the safety and immunogenicity to PXVX0317 in adults ≥65 years of age.

Co-primary Objectives:

* To compare the anti-CHIKV serum neutralizing antibody (SNA) response to PXVX0317 and placebo at Day 22, as measured by geometric mean titer (GMT) and clinically relevant difference in seroresponse rate (PXVX0317 minus placebo) in adults ≥65 years of age.
* To evaluate the safety of PXVX0317 in adults ≥65 years of age

Secondary Objectives:

* To compare the anti-CHIKV SNA response to PXVX0317 and placebo at Day 15 and Day 183, as measured by GMT and seroresponse rate.
* To compare the anti-CHIKV SNA response to PXVX0317 and placebo in participants ≥65 to \<75 and ≥75 years of age as measured by GMT and seroresponse rate.

Eligibility Criteria:
Inclusion Criteria:

* Able and willing to provide informed consent voluntarily signed by participant. Must verbalize understanding of the general procedures of, and reason for the study.
* Males or females, ≥65 years of age.
* Able to complete all scheduled visits and comply with all study procedures.
* Women who are not of childbearing potential (CBP): surgically sterile (at least six weeks post bilateral tubal ligation, bilateral oophorectomy or hysterectomy); or postmenopausal (defined as a history of ≥12 consecutive months without menses prior to randomization in the absence of other pathologic or physiologic causes, following cessation of exogenous post menopausal sex-hormonal treatment).
* Participants must be in stable health in the opinion of the investigator for at least 30 days prior to screening (eg, no hospital admission for acute illness in the last 30 days prior to screening).

Exclusion Criteria:

* Participation or planned participation in an investigational clinical trial (eg, vaccine, drug, medical device, or medical procedure) within 30 days of Day 1 and for the duration of the study. Note: Participation in an observational trial or follow-up phase of a trial may be allowed; however, these instances should be discussed with the sponsor's medical monitor (MM) prior to enrollment.
* Prior receipt of any CHIKV vaccine.
* Positive laboratory evidence of current infection with human immunodeficiency virus (HIV), hepatitis C virus (HCV) or hepatitis B virus (HBV).
* Body mass index (BMI) ≥35 kg/m\^2
* History of any known or suspected allergy or history of anaphylaxis to any component of the investigational product (IP).
* History of any known congenital or acquired immunodeficiency or immunosuppressive condition that could impact response to vaccination (eg, leukemia, lymphoma, malignancy, functional or anatomic asplenia, alcoholic cirrhosis). Note: History of basal cell and squamous cell carcinoma of the skin or carcinoma in situ of the cervix considered cured would not be exclusionary. History of a malignancy considered cured from over five years from the date of screening with minimal risk of recurrence is not exclusionary.
* Prior or anticipated use of systemic immunomodulatory or immunosuppressive medications from six months prior to screening through Day 22. Note: Systemic corticosteroid use at a dose or equivalent dose of 20 mg of prednisone daily for 14 days or more within 90 days of screening through Day 22 is exclusionary. The use of inhaled, intranasal, topical, or ocular steroids is allowed.
* Bleeding disorder or receipt of anticoagulants in the 21 days prior to screening, contraindicating intramuscular (IM) vaccination, as judged by the investigator.
* Moderate or severe acute illness with or without fever (oral temperature ≥100.4°F or 38.0°C).
* Receipt or anticipated receipt of immunoglobulin from 180 days prior to screening through Day 22.
* Medical condition (such as dementia) that, in the opinion of the investigator, could adversely impact the participant's participation in or conduct of the study.
* Evidence of substance abuse that, in the opinion of the investigator, could adversely impact the participant's participation in or conduct of the study.
* Identified as an investigator or employee of an Investigator or study center with direct involvement in the proposed study, or identified as an immediate family member (ie, parent, spouse) of the investigator or employee with direct involvement in the proposed study.
* Receipt or anticipated receipt of any vaccine from 30 days prior to Day 1 through Day 22.
* Receipt or anticipated receipt of blood or blood-derived products from 90 days prior to screening through Day 22.
* Any planned elective surgery that may interfere with study participation or conduct.
* Any other medical condition that, in the opinion of the investigator, could adversely impact the participant's participation in or conduct of the study.