Long-term Follow-up Study to Evaluate Safety and Immunogenicity of CHIKV VLP Single or Booster Vaccination

The purpose of this phase 3 multicenter, randomized, double-blind, placebo-controlled rollover study is to evaluate the safety and long-term immunogenicity of CHIKV VLP vaccine in adult and adolescent participants and to evaluate CHIKV VLP booster vaccine induced serum neutralizing antibody (SNA) response at 3 or 4 years post-initial CHIKV VLP vaccination.

Inclusion Criteria: * Only within the EBSI-CV-317-004 feeder study informed consent form (ICF) (and/or assent form, as applicable), the participant voluntarily signed and agreed to be contacted or did not indicate they were not to be contacted for potential screening and enrollment in a future study (ie, EBSI-CV-317-008). * Able and willing to provide informed consent (and assent, as applicable) voluntarily signed by participant (and guardian, as applicable) for participation in this rollover study EBSI-CV-317-008, including possible receipt of a booster dose of CHIKV VLP vaccine. * Males or females, 12 years of age or older at the time of enrollment in the feeder study. * Received a single dose of CHIKV VLP vaccine in one of the feeder studies, EBSI-CV-317-004 or EBSI-CV-317-005. * Demonstrated compliance to the feeder study conduct (ie, rollover participant was without protocol deviations that excluded them from immunogenicity analysis in feeder study EBSI-CV-317-004 or EBSI-CV-317-005) without discontinuation or early withdrawal. * Generally healthy, in the opinion of the investigator, based on medical history and physical examination. Additional inclusion criteria to be assessed at Prerandomization Visit (Visit 5), before Randomization A (Visit 6), and before Randomization B (Visit 8 for Group 2) to determine eligibility for a booster dose of CHIKV VLP vaccine or placebo: \- Women who are either: i. Not of childbearing potential (CBP): premenarche, surgically sterile (at least six weeks postbilateral tubal ligation or bilateral total salpingectomy, 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 sex-hormonal treatment). For women who are postmenopausal, documented follicle stimulating hormone (FSH) level of ≥40 mIU/mL must be obtained. If the FSH is \