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

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.