Inclusion Criteria:

• Subjects who the investigator believes that they and/or their parents/guardians can and will comply with the requirements of the protocol (e.g., completion of the diary cards, return for follow-up visits) should be enrolled in the study.
• A healthy male or non-pregnant female between 12 months (mths) and 50 years (yrs) of age at the time of the first vaccination;
• Free of obvious health problems as established by medical history and physical examination before entering into the study;
• For children: 23mths of age, full compliance with the United States Advisory Committee on Immunization Practices (U.S. ACIP) recommended childhood immunization schedule;
• Written informed consent obtained from the subject or a parent/guardian and assent for subjects 7-20 yrs of age;
• If the subject is female, she must be of non-childbearing potential, i.e. either pre-menarcheal, surgically sterilized or one year post-menopausal; or, if of childbearing potential, she must be abstinent or have used adequate contraceptive precautions (i.e. intrauterine contraceptive device; condom and spermicide combination, oral contraceptives or other equivalent hormonal contraception, e.g. progestin implantable, cutaneous hormonal patch or injectable contraceptives) for 30 days (dys) prior to vaccination, have a negative pregnancy test within 48 hrs prior to vaccination and must agree to continue such precautions for 60 dys after completion of the vaccination series. Any child who begins menarche during the study period must follow the same precautions listed above, from menarche until 60 dys after the second vaccine dose.

Exclusion Criteria:

• Pregnant or lactating female;
• Female planning to become pregnant or planning to discontinue abstinence or contraceptive precautions;
• History of any neurological or behavioral disorder or seizures, with the exception of a single febrile seizure in childhood;
• History of allergic disease/reaction likely to be exacerbated by any component of the vaccine;
• Acute or chronic, clinically significant pulmonary, cardiovascular, hepatic, renal, hematologic or endocrine functional defect, as determined by physical examination or laboratory tests;
• Any confirmed or suspected immunosuppressive or immunodeficient condition;
• Acute disease at the time of enrollment (acute disease is defined as the presence of a moderate or severe illness with or without fever); note that vaccine can be administered to persons with a minor illness such as diarrhea, mild upper respiratory infection with or without low-grade febrile illness, i.e., equivalent to an oral temperature <37.5°C/<99.5°F.
• Chronic hepatomegaly, right upper quadrant abdominal pain or tenderness;
• Chronic splenomegaly, left upper quadrant abdominal pain or tenderness;
• Use of any investigational or non-registered drug or vaccine other than the study vaccine within 30 dys preceding the first dose of study vaccine/placebo or planned use during the study period;
• Planned administration of a vaccine not foreseen by the study protocol during the period starting from 30 dys before each dose of the study vaccine and ending 30 dys after; with the exception of standard infant and children "inactivated" vaccines or the inactivated influenza vaccine administered to adults or children;
• A planned move to a location that will prohibit participating in the trial for the 12 mth duration;
• Chronic administration (defined as more than 14 dys) of immunosuppressants or other immune-modifying drugs within 90 dys preceding the first dose or planned administration during the study period. For corticosteroids, this will mean prednisone, or equivalent, 0.5 mg/kg/day. Inhaled and topical steroids are allowed;
• Administration of immunoglobulins and/or blood products within 90 dys preceding the first dose or planned administration during the study period;
• Hypertension;
• Chest pain, palpitations, dizziness, shortness of breath unrelated to asthma, arrhythmias or friction rubs;
• Any chronic systemic drug therapy to be continued during the study period (except for vitamin/mineral supplements, routine treatment for gastro-esophageal reflux);
• Potential adult volunteers, or parents of potential child volunteers, who do not have easy access to a fixed or mobile telephone;
• History of chronic alcohol consumption and/or drug abuse.