Outbreaks of yellow fever continue to erupt despite availability of an effective vaccine since the 1930s
Geographic focus of the disease shifting from West to Central and East Africa
UP TO 60,000 YELLOW FEVER DEATHS EVERY YEAR
Before the development of a life-saving vaccine in the 1930s, the yellow fever virus was responsible for devastating epidemics in large cities in Africa, the Americas and Europe.
In the last 20 years, a combination of declining population immunity, rapid urban migration, climate change, deforestation and an increase in mosquito vectors has led to a resurgence of yellow fever. Forty-seven countries in Africa (34) and Central and South America (13) are endemic, or have regions that are endemic, for yellow fever. A recent modelling study estimated that yellow fever infects between 840,000 and 1.7 million people in Africa each year, resulting in 84,000–170,000 cases and 29,000–60,000 deaths.1
Rapid urbanisation has exacerbated the issue by concentrating people who have not been immunised in settings where the yellow fever virus thrives. City areas provide fertile breeding grounds for mosquito larvae, as stagnant water collects in water containers, cans and tyres. Overcrowded housing further accelerates the spread of the virus.
An acute viral haemorrhagic disease transmitted by mosquitoes, yellow fever causes devastating epidemics in areas where infected mosquitoes can come in contact with people who are not vaccinated. Death rates can be as high as 50% among those severely affected.
Yellow fever can be prevented by a safe, affordable and highly effective vaccine. One injection protects an individual for at least 35 years, and possibly for life.
Yellow fever cannot be eliminated. However, the risk of outbreaks can be substantially reduced by immunising at least 70% of the at-risk population. In order to achieve this high coverage and maintain immunity for life in the 34 endemic countries in Africa, Gavi supports routine immunisation and preventive campaigns in eligible countries.
The recent resurgence of yellow fever is especially pronounced in West and Central Africa, where mass vaccination campaigns in the period 1933–61 effectively meant the virus had disappeared.
This alarming trend started in equatorial Africa with a 1990 epidemic in Cameroon, and then spread across the region. By 2005, failure to immunise successive birth cohorts through routine immunisation had led to approximately 200,000 yellow fever cases and more than 50,000 deaths in West Africa's 12 highest-risk countries.
Today, the virus poses the greatest threat to 900 million people in more than 47 endemic countries – 34 in Africa and 13 in Central and South America. Shifts in migration patterns and environmental changes are affecting the geography of yellow fever, with outbreaks now occurring in areas historically considered non-endemic.
In 2015, two “low-risk” central African countries reported yellow fever cases – Equatorial Guinea and Gabon. In December, the initial cases of an urban outbreak were detected in Luanda, Angola – the beginning of an outbreak that subsequently spread throughout the community as well as outside of Africa.
Faced with an increasing risk of large urban yellow fever outbreaks, WHO has revised its long-term yellow fever control strategy. This emphasises the importance of international health regulations in preventing cross-border spread of the virus, and the need to enhance surveillance to detect, confirm and respond to yellow fever cases in a timely way. The strategy is closely aligned with previous Gavi-funded yellow fever immunisation strategies.
SHORTAGE OF SUPPLY
Even though four companies manufacture the yellow fever vaccine, the demands of mass preventive campaigns and outbreak response have severely depleted supplies. Doses intended for preventive campaigns are frequently diverted to treat outbreaks, leaving emergency stocks dangerously low and delaying preventive campaigns. In both Nigeria and the Sudan, for example, campaigns are three–four years overdue. In response, Gavi has worked with WHO to revise its yellow fever control strategy.