A Study to Investigate the Safety and Immunogenicity of the SF2a-TT15 Synthetic Carbohydrate-based Conjugate Vaccine Against Shigella Flexneri 2a

Inclusion Criteria: For adults: 1. Healthy men and women between 18 and 50 (inclusive) years of age. 2. Subjects who provide written informed consent or thumb print in the presence of a witness to participate in the study 3. Women willing to use at least 1 reliable method of contraception during the study period, or are surgically sterilized, and agree to undergo repeated pregnancy tests (before each vaccination) and men willing to use an effective method of contraception (e.g. condom). For children and infants: 1. Healthy boys and girls between 2 and 5 years of age for the children group (cohort 2) 2. Healthy boys and girls 9 mo-old (+/- 1 month) for the infant group (cohort 3) 3. Parents or legally acceptable representatives, as appropriate, who are willing and able to provide signed/thumb printed informed consent for children and infants. 4. Infant and children should have a normal nutritional Z score (-2 or greater) according to the mother and child health handbook of the republic of Kenya - Ministry of Health before entering the trial. For all: 1. Signed/thumb written informed consent, in accordance with local practice, provided by adult volunteers (participants 18 years of age and older), parent(s) or legal representative(s) for children and infants participants as applicable, who, in the opinion of the Investigator, can and will comply with the requirements of the protocol. 2. Subjects in general good health in the opinion of the Investigator as determined by medical history, vital signs and a physical examination. 3. No clinically significant abnormalities in hematology, blood chemistry, or urinalysis laboratory tests at screening. 4. Negative HIV, Hepatitis B and Hepatitis C serology tests and malaria test. Exclusion Criteria: 1. Subjects with a history of clinically significant gastrointestinal disorders (e.g. gastroesophageal reflux disease, peptic ulcer, celiac disease, inflammatory bowel disease). 2. Individuals with immunosuppressive diseases or under immunosuppressive therapy. 3. Previous participation in any study in which a Shigella-vaccine candidate was administered. 4. Suspected or known hypersensitivity (including allergy) to any of the vaccine components or to previous vaccine, or to medicinal products or medical equipment whose use is foreseen in this study. 5. Use of any prescription or over-the-counter (OTC) medications, within 14 days prior vaccination. Paracetamol or ibuprofen for symptomatic relief of pain is allowed until 48 hours prior to vaccination. 6. Women who are pregnant, breast-feeding, or are of childbearing age and are not on or do not plan to use acceptable contraceptives for the duration of the study. 7. Subjects with any significant acute medical situation (e.g. acute infection) within 48 hrs prior to study entry, in the opinion of the Principal Investigator. 8. Participation in another clinical trial with drugs within 3 months prior first study injection.