Yellow fever (YF) is a mosquito-borne viral disease that is endemic to sub-Saharan Africa and tropical South America. YF virus causes an estimated 200,000 cases of clinical disease and 30,000 deaths annually [WHO 1992]. Clinical disease ranges from a mild, undifferentiated febrile illness to severe disease with jaundice and hemorrhage. The case-fatality ratio for severe YF is 20%-50% [Monath 2013]. Because no specific treatment exists for YF, prevention is critical to reduce disease risk. One of the most effective prevention measures against YF is vaccination with the live, attenuated YF 17D substrain virus vaccine. YF vaccine is recommended for persons aged ≥9 months who are traveling to or living in areas with risk for YF virus transmission [CDC 2010]. In addition, International Health Regulations allow countries to require proof of YF vaccination from travelers entering their country [WHO 2005]. These requirements are intended to minimize the potential importation and spread of YF virus. Proof of YF vaccination is recorded on the International Certificate of Vaccination or Prophylaxis (i.e., yellow card). International Health Regulations specify that the International Certificate of Vaccination or Prophylaxis is valid for 10 years. Therefore, if 10 or more years have elapsed since the last vaccination, people planning travel to a country with a YF vaccination entry requirement need to receive a booster dose of the vaccine. The Strategic Advisory Group of Experts on Immunization (SAGE), the principal advisory group to the World Health Organization (WHO) for vaccines and immunization, concluded in April 2013 that a single dose of YF vaccine is sufficient to confer sustained immunity and lifelong protection against YF disease, and a booster dose of the vaccine is not needed [WHO 2013]. This conclusion was based on a systematic review of published studies on the duration of immunity following a single dose of YF vaccine, and on data that suggest vaccine failures are extremely rare and do not increase in frequency with time since vaccination [Gotuzzo 2013]. SAGE noted that future studies and surveillance data should be used to identify specific risk groups, such as infants or persons infected with human immunodeficiency virus (HIV), who could benefit from a booster dose. In May 2014, the World Health Assembly adopted the recommendation to remove the 10 year booster dose requirement from the International Health Regulations by June 2016 [WHO 2014]. In the United States, the current ACIP YF vaccine recommendations note that “[International Health Regulations] require revaccination at intervals of 10 years to boost antibody titer. Evidence from multiple studies demonstrates that YF vaccine immunity persists for many decades and might provide life-long protection” [CDC 2010]. The ACIP Japanese Encephalitis Vaccine Work Group was reformed to include YF vaccine in October 2013 to discuss the need for booster doses of YF vaccine.