Long-term Follow-up Study to Evaluate Safety and Immunogenicity of PXVX0317 Single or Booster Vaccination

Primary Objectives:

* To evaluate the long-term immunogenicity of CHIKV VLP vaccine in healthy adult and adolescent participants as measured by proportion of participants maintaining an anti-CHIKV serum neutralizing antibody (SNA) titer ≥100 (seroresponse rate, also considered the presumptive seroprotection rate) at yearly intervals up to 5 years postvaccination in feeder studies EBSI-CV-317-004 (NCT05072080) and EBSI-CV-317-005 (NCT05349617). * To assess the vaccine-induced SNA titers by a booster dose of CHIKV VLP vaccine at 3, 4, or 5 years post-initial vaccination in feeder studies EBSI-CV- 317-004 and EBSI-CV-317-005. * To evaluate the safety and tolerability of CHIKV VLP vaccine in all participants. * To evaluate the safety and tolerability of a booster vaccination and compare with safety and tolerability reported post-initial vaccination of CHIKV VLP vaccine under feeder studies EBSI-CV-317-004 and EBSI-CV-317-005 in healthy adults and adolescents.

Secondary Objectives:

* To evaluate the long-term immunogenicity of CHIKV VLP vaccine in healthy adult and adolescent participants as measured by anti-CHIKV SNA geometric mean titers (GMTs) at yearly intervals up to 5 years post-initial vaccination in feeder studies EBSI-CV-317-004 and EBSI-CV-317-005. * To evaluate the immune response to a booster vaccination and compare this response to that reported post-initial vaccination of CHIKV VLP vaccine under feeder studies EBSI-CV-317-004 and EBSI-CV-317-005 in healthy adults and adolescents.