A Study of Live Oral Cholera Vaccine, PXVX200 in Healthy Older Adults

Inclusion Criteria: 1. Able to understand the study and give written consent. 2. Healthy male and female adults, age 46-64 years (inclusive) without significant medical history, physical, or abnormal screening laboratory test results at screening. 3. Women of childbearing potential must have had a negative urine pregnancy test at screening, prior to vaccination. Female subjects must be of non-childbearing potential (as defined as surgically sterile or postmenopausal for more than 1 year), or if of childbearing potential must be practicing abstinence or using an effective licensed method of birth control (eg, use hormonal or barrier birth control such as implants, injectables, combined oral contraceptives, intrauterine devices \[IUDs\], cervical sponges, diaphragms, condoms with spermicidal agents; or must have a vasectomized partner) within 2 months of vaccination and must agree to continue such precautions during the study. 4. Willing and able to comply with the study requirements and procedures. Exclusion Criteria: 1. Currently active unstable or undiagnosed medical conditions including immunodeficiency, cardiovascular disease, respiratory disease, endocrine disorder, liver disease, renal disease, gastrointestinal disease, neurologic illness, psychiatric disorder requiring hospitalization, current drug or alcohol abuse. Examples of unstable or undiagnosed medical conditions including unstable angina pectoris, shortness of breath on exertion without clear etiology and chronic renal failure requiring dialysis. Examples of conditions that do not meet exclusion criteria include mild controlled hypertension, mild controlled asthma, and treated depression without hospitalization. 2. Abnormal stool pattern defined as fewer than 3 stools per week or more than 2 stools per day in past 6 months. 3. Regular use of laxatives in the past 6 months. 4. Previously received a licensed or investigational cholera vaccine. 5. History of cholera or enterotoxigenic E. coli infection (natural infection or experimental challenge). 6. Travel to a cholera-endemic area in the previous 5 years. 7. Received or plans to receive any other licensed vaccines, except for seasonal influenza vaccine, from 14 days prior to the study vaccination through to 29 days after vaccination. 8. Received or plans to receive antibiotics or chloroquine within 14 days prior to the study vaccination through to 29 days after vaccination. 9. Recipient of bone marrow or solid organ transplant. 10. Use of systemic chemotherapy in the previous 5 years prior to the study. 11. Malignancy (excluding non-melanotic skin cancers) or lymphoproliferative disorders diagnosed or treated during the past 5 years. 12. Received or plans to receive systemic immunosuppressive therapy, radiation therapy, parenteral or high-dosage inhaled steroids (\>800 µg/day of beclomethasone diproprionate or equivalent) within 6 months prior to the study vaccination through to Day 29. 13. History of Guillain-Barré Syndrome. 14. Pregnant or nursing.