Booster Dose (50 µg) of Recombinant Ricin Toxin Vaccine (RVEc™) in Previously Vaccinated Healthy Adults

Inclusion Criteria: * Study subjects must have received the 3 priming doses of RVEc at the 50-µg dosage. * Study subjects must be 18 years old at time of screening and no older than 50 years old at time of vaccination. * Subjects must weigh at least 110 pounds at time of screening. * Subjects must be in good health as judged from medical history, physical examination, EKG, hematology, clinical chemistries, urinalysis, hepatitis serology, and HIV antibody test (Table 4) and be medically cleared for participation by an investigator. If any HIV or hepatitis testing is positive, the individual will be provided with counseling and referral for health care. * Females of child-bearing potential must have a negative pregnancy test on screening and the morning of vaccination prior to receipt of the vaccine and must agree to use a highly effective method of birth control during the first 3 months following receipt of RVEc. A highly effective method of birth control is defined as one with a failure rate of less than 1% per year. Acceptable birth control methods that meet this criterion include hormonal implants and injectables (Norplant, Depo-Provera, Lunelle, and Etonogestrel), combined oral contraceptives, the intrauterine devices (IUDs) Copper T (380-A) or Mirena (Levonorgestrel Intrauterine System), female sterilization (tubal ligation), sexual abstinence, or a vasectomized partner. * Female subjects must also agree not to breastfeed until at least 3 months after receiving the RVEc. * Study subjects must read and sign an approved informed consent. * Study subjects must be willing to return for all follow-up visits. * Study subjects must agree to report any AE that may or may not be associated with administration of the investigational product through the 6-month follow-up/closeout visit. * Study subjects must agree to abstain from excessive exercise (more than the usual routine) and excessive alcohol consumption (exceeds more than 2 drinks for males or 1 drink for females daily or binge drinking) for the duration of study participation. Exclusion Criteria: * Acute or chronic medical conditions or immunodeficiency from a medical condition or medical treatment, medications, or dietary supplements that, in the investigator's opinion, would impair the subject's ability to respond to vaccination. Use of corticosteroids, other than inhaled corticosteroids, will not be permitted. * Severe hypersensitivity to any vaccine (anaphylaxis, angioedema, bronchospasm, or laryngospasm). * History of asthma, chronic obstructive pulmonary disease, or other current/residual diseases of the lungs. * Clinically significant abnormal laboratory tests. * Current smoker. * Any known allergies to sodium succinate, Tween-20 (a detergent), aluminum hydroxide, or kanamycin. * Receipt of any vaccine or investigational product within 30 days before or after vaccination with RVEc. * Receipt of RVEc is contraindicated based on SIRVA checklist/screening, as determined by PI.