African nations. 1 A recent genetic analysis of yellow fever virus isolates from 22 countries suggests that the virus most likely first appeared in East Africa, was imported into the Americas from West Africa, and then spread westward across the Americas. 2 In South America, Bolivia, Brazil, Columbia, Ecuador, Peru and Venezuela are considered to be at greatest risk. The disease is characterized by variable symptoms, ranging from flu-like illness to a rapid evolution of severe hepatitis, renal failure, hemorrhage, shock and death. Mortality rates vary between 20% and 100%. 3 No specific treatment for YF is available; only supportive care can be offered. Vaccination is recognized as the most effective means of controlling yellow fever. 4 Highly effective 17D YF vaccines have been available for nearly 70 years; two live, attenuated substrains, 17D-204 and 17DD, are used in currently available vaccines. 5,6 The 17DD vaccine is used predominantly in South America while 17D-204 vaccines are used throughout the world. Seroconversion occurs within ten days in over 95% of people vaccinated. A single dose of vaccine provides protection for at least 10 years. 7 Although about 500 million doses of YF vaccine have been administered world wide, an estimated 200,000 cases of YF, including 30,000 deaths, still occur each year. The risk of contracting the disease is highest in Africa, where about 90% of cases occur, with the remaining 10% in South America. 7 In both South America and Africa, it is likely that only a small proportion of cases are officially recorded, and recognition of outbreaks is often delayed because YF often occurs in remote jungle areas. In addition, diagnostic facilities are lacking. Investigations of outbreaks in endemic areas of Africa have shown that, during epidemics, 20-40% of the population have serologic evidence of infection. Overt severe disease is seen in 3-5% of these cases, and the case-fatality rate (CFR) ranges from 20% to 60.% 7,8 Epidemiologic estimates from Peru for the years 2004-2005 indicate an annual YF incidence of approximately 18/100,000 for an at risk population of 320 to 350,000 people. The CFR during those years was estimated to be 46-51%. 9 Whether regional differences reflect reporting artifacts or a difference in virus strain virulence and/or genetic susceptibility of
