Over the last 10 years, there has been an increase in the number of countries reporting yellow fever to WHO, especially in West Africa where 93 % of the countries notified cases in the past 4 years, a 30% increase compared to the period 1995-1999. This reveals a worrying increase in the circulation of the virus in a non-immune human population. The geographical spread of the virus is also disconcerting. Indeed, multiple outbreaks may occur simultaneously in several different places, stressing the response capacity of the country as well as the support capabilities of the international community.
Historically, when yellow fever vaccine was not available, small outbreaks of yellow fever that occurred in a non-immune population in densely populated cities were often forerunners of large urban epidemics.
Today, 12 countries with large non-immune populations are at high risk, and immunization needs to be intensified now. For example, the last yellow fever outbreak in Nigeria occurred 14 years ago, but it took 10 years to control the transmission of the virus in the population. Given the low vaccine coverage of children under 14 years old, the number of children at risk in Nigeria has been estimated at 23 million, for those children in urban areas only. Immunizing the urban populations in these high-risk countries would require vaccinating approximately 100 million people.