Long-term Follow-up Study to Evaluate Safety and Immunogenicity of PXVX0317 Single or Booster 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 and Visit 10 for Group 3) to determine eligibility for a booster dose of CHIKV VLP vaccine or placebo: \- Women who are either: i. Not of childbearing potential (CBP): premenar…