Increased risk of urban yellow fever outbreaks in Africa
The risk of large and uncontrollable outbreaks in urban areas in Africa is more likely than ever. Accelerated urbanization has concentrated a non-immune population in settings where, high vector and population density, the main factors contributing to increased virus transmission are present.
While 62.1% of the African population is still rural, urban growth rates at nearly 4% a year are the most rapid in the world, and nearly twice the global average1. Not only are more people living in cities, but the cities themselves are becoming larger and more numerous. There are now 43 cities in Africa with more than one million people, a figure which is expected to increase to almost 70 by 20152.
Data from 2000, shows that the population of Lagos, Nigeria increased at a rate of 21 persons per hour; between 1994 and 2004, urban population growth rates in West African cities ranged from between 10% and 39%, with an average of 18%. By 2020, the proportion of urban population to rural population will reach 63% in sub-Saharan Africa.3
Many African cities now have an increasing number of overcrowded, informal settlements, or 'shanty towns', characterized by low-grade housing, poor roads, inadequate water supplies, sanitation and waste management services. Most people who live here have no access to running water and store drinking water in containers which often serve as breeding sites for the mosquito Aedes aegypti, the primary vector of urban yellow fever. In addition, the lack of public sanitation services in many large cities prevent the removal of other artificial breeding sites such as metal cans, tires or derelict vehicles.
As a result of the increased virus circulation in West Africa and intense population migration from infected forest areas to urban settings, five large cities have already faced yellow fever epidemics in the past four years. Many smaller cities are still exposed to the disease.
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.
High levels of resources are needed to control an urban outbreak, depriving countries of scarce resources to control other diseases and to improve health services. In 2001, during the yellow fever epidemic in Abidjan, Côte d'Ivoire, 2.61 million people were vaccinated in 12 days. During the same period a cholera outbreak occurred, resulting in 3250 cases and 59 deaths. Most of the national and international support went to controlling yellow fever, with few resources left to combat the cholera outbreak.
With the rise of rapid, global transport and communication links, the risk of international spread of yellow fever from African cities has grown. Although the virus circulation has remained within historically endemic areas for now, the virus could spread quickly to those areas with a high density of vectors.
The potential for a major urban outbreak of yellow fever in Africa is already present. It is critical that a sustained response start today.
Population of main cities in Africa
1United Nations, Population Division, 2001
2Meeting the Urban Challenge, Population Reports Series M, number 16 United Nations, World Urbanization prospects, the 1999 revision
3United Nations, World Urbanization Prospects, the 1999 revision