Training Study to Characterize Biomarkers to Chickenpox and Yellow Fever Vaccines

Inclusion Criteria: 1. Healthy male or female subjects aged 18-45years inclusive. 2. The subject is, in the opinion of the investigator, healthy on the basis of medical history, vital signs, \& the results of routine laboratory tests with no active disease process that could interfere with the study endpoints. 3. Body Mass Index ≥18.5 and \<29.5 4. The subject is able to read \& understand the ICF, \& understand study procedures. 5. The subject has signed the ICF. 6. Subjects must fulfill the immune status eligibility only for the group they are allocated to:: 1. To be eligible for the Varilrix group the subject must be immune to varicella zoster confirmed on screening by a varicella zoster serum antibody titre by ELISA test (cut-off: 25 mIU/mL) 2. To be eligible for the Stamaril group the subject must be seronegative to flaviviruses as confirmed by ELISA test with a predetermined cut-off 7. Available for followup for the duration of the study. 8. Agree to abstain from donating blood during \& for three months after the end of their participation in the study, or longer if necessary. 9. If heterosexually active female, willing to use an effective method of contraception with partner from 30 days prior to, \& 3 months after, vaccination. Willing to undergo urine pregnancy tests prior to vaccination \& blood pregnancy test at screening \& final follow up. 10. The subject has venous access sufficient to allow blood sampling as per protocol. Exclusion Criteria: 1. Significant dietary restrictions (e.g. vegan, lactose intolerant, but vegetarian acceptable) or life threatening food allergies (e.g. anaphylaxis related nut allergies). 2. Pregnant or lactating at any point during the study from screening to final follow up. 3. Subjects must fulfill the vaccine contraindications eligibility only for the group they are allocated to: a.For group A (Varilrix) - i.History of hypersensitivity to neomycin (other than contact dermatitis), any of the excipients in the vaccine (amino acids, human albumin, lactose, mannitol, sorbitol) or to any other varicella vaccine. ii.Known hypersensitivity to a first dose of Varilrix. b.For group B (Stamaril) - i.Previous receipt of a yellow fever vaccine ii.History of hypersensitivity to eggs, chicken proteins or any component of Stamaril (Lactose, Sorbitol E420, L-histidine hydrochloride, L-alanine, Sodium chloride, Potassium chloride, Disodium phosphate, Monopotassium phosphate, Calcium chloride, Magnesium sulphate) 4. Presence of primary or acquired immunodeficiency states with a total lymphocyte count less than 1,200 per mm3 or presenting other evidence of lack of cellular immune competence e.g. leukaemias, lymphomas, blood dyscrasias, or patients receiving immunosuppressive therapy (including regular use of oral, inhaled, topical or parenteral corticosteroids). 5. Use of any immune suppressing or immunomodulating drugs within 6 months of Visit 1 6. Regular use of nonsteroidal anti-inflammatory drugs (by any route of administration including topical) within 6 months of Visit 1 (screening) considered by the study physician as likely to interfere with immune responses. 7. Receipt of a vaccine within 30 days of visit 2, or requirement to receive another vaccine within the study period. 8. Presence of an acute severe febrile illness at time of immunization. 9. History of alcohol, narcotic, benzodiazepine, or other substance abuse or dependence within the 12 months preceding Visit 1. 10. Currently participating in another clinical study with an investigational or non-investigational drug or device, or has participated in a clinical trial within the 3 months preceding Visit 1. 11. Any condition that, in the investigator's opinion, compromises the subject's ability to meet protocol requirements or to complete the study. 12. Receipt of blood products or immunoglobin, or blood donation, within 3 months of screening. 13. Unable to read \& speak English to a fluency level adequate for the full comprehension of procedures required in participation \& consent. 14. An average weekly alcohol intake that exceeds 14 or 21 units per week for females and males, respectively, or unwilling to stop alcohol consumption for each treatment period during the study. 15. Currently smokes in excess of 5 cigarettes/day or equivalent use of tobacco (within the last 6 months of screening), or subjects unwilling to refrain from smoking or are unable to abide by Surrey CRC restrictions. 16. Consumes excessive amounts, defined as greater than 4 servings of coffee, tea, cola, or other caffeinated beverages/food per day.