Yellow fever virus is found in tropical and subtropical, and caused an estimated 29,000 to 60,000 deaths in 2013. The National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health, has begun an early-stage clinical trial of an investigational vaccine designed to protect against yellow fever virus. The Phase 1 study is evaluating whether an experimental vaccine developed by the Danish biopharmaceutical company Bavarian Nordic is safe, tolerable and has the potential to prevent yellow fever virus infection.