Safety and Immunogenicity of Venezuelan Equine Encephalomyelitis Vaccine (VEE C-84) as a Booster to VEE TC-83

Inclusion Criteria: * At least 18 years old. * VEE PRNT80 \< 1:20 before immunization. * (females) Negative urine pregnancy test on the same day before vaccination. Not planning pregnancy for 3 months. * Actively enrolled in the SIP. * At risk for exposure to virulent VEE virus (with up-to-date risk assessment). * Previous TC-83 vaccination * Up-to-date (within 1 year) physical examination/tests. * Sign and date the approved informed consent. * Willing to return for all follow-up visits. * Agree to report adverse event (AE) up to 28 days after vaccination. Exclusion Criteria: * Over age of 65 years * Clinically significant abnormal lab results including evidence of Hepatitis C, Hepatitis B carrier state, or elevated liver function tests. * History of immunodeficiency or current treatment with immunosuppressive medication. * (females) Currently breastfeeding. * Confirmed human immunodeficiency virus (HIV) titer. * Any known allergies to components of the vaccine. * A medical condition that in the judgment of the Principal Investigator (PI) would impact subject safety (i.e-vaccination and or exposure to another alphavirus). * Administration of any vaccine within 28 days of C-84. * Any unresolved AEs resulting from a previous immunization.