Last week, the World Health Organization (WHO) reported one of its largest emergency vaccination campaigns for the yellow fever virus in Africa – in Angola in southern Africa and its northern neighbour, the Democratic Republic of Congo. The outbreak has already claimed more than 400 lives and sickened thousands more.
This is also the first time such a large outbreak of yellow fever has needed to be controlled in densely populated urban regions. Health workers in the region now have the enormous task of vaccinating more than 17 million people before the rainy season starts in September, which presents ideal breeding conditions for the mosquitoes that spread the virus.
The yellow fever virus is transmitted by two species of mosquitoes of which the main type, Aedes aegypti, thrives in urban environments. Although most people experience either minor symptoms (such as fever, muscle ache and nausea) or none at all, a small minority of those infected go on to enter a second, more toxic disease stage. This is marked by serious symptoms such as jaundice (yellowing of the skin and whites of the eyes), kidney failure and bleeding from the mouth, nose, eyes or stomach. Up to half of people that experience these more serious symptoms will die.
Great challenges ahead
During the course of the Zika epidemic in South America, labs around the world have been in a race to come with an effective vaccine. Yet for the yellow fever virus, a safe and effective vaccine has been available since the 1930s – a single dose or two can protect an individual for the rest of their lives. In 1951, the Nobel Prize in Medicine or Physiology was given to Max Theiler for the development of the vaccine and so far this remains the only time that a Prize has been awarded for the development of a vaccine.
However, the current global production of yellow fever vaccine can’t sustain the present rate of vaccination needed to give everyone at risk a single dose. This is partly attributed to a year-long laborious process of manufacturing the vaccines in pathogen-free eggs. Since December 2015, 19 million doses of the vaccine has already been given to people in affected areas, and now the global emergency stockpile is critically low with only 5 million doses remaining. Angola alone has a population over 21 million people. Prior to the report of the first outbreak there in December 2015, the country was not considered at risk so most people are not vaccinated. Therefore, the WHO plan to dilute the vaccine fivefold in the hope that it will provide shorter-term protection for 12 months to as many people as possible.
Aside of the logistics from carrying out such mass immunisation programme, there is an added problem of storing the vaccines. Yellow fever vaccines need to be refrigerated with ice packs due to the lack of reliable electricity and sources of fuels for generators.
Clearly having a safe and effective vaccine is only the first step towards combating mosquito borne diseases. With accelerated urbanisation in Africa and the growing emergence of mosquitoes such as Aedes aegypti that thrive in urban environments, the world needs to develop strategies that allow us to monitor and respond quickly and effectively to such disease outbreaks.
Yeping Lu, Communications Intern, British Society for Immunology