Booster Vaccine for Yellow Fever

Inclusion Criteria: * Any child fitting the required age cohorts who has a documented record of having received a primary dose of the Institut Pasteur YF 17D vaccine between 9 and 12 months of age. * Documented evidence can be either a previous record of YF vaccination with dates in our own trial registers if the child was a previous participant or documented evidence on the Infant Welfare card. Exclusion Criteria: * Any child with a height/length for weight z-score of -3 or below. * Any child known to be immunocompromised including any child with know vertical exposure to HIV infection. * Any child with a history of serious adverse event or other contraindication to previous yellow fever vaccination. * Participants who have an acute illness including abnormal vital signs or a fever of \> 37.5°C will not be vaccinated on the day but may be invited back for rescreening when they have recovered.