While a World Health Organization-led strategy aims to provide 1.4 billion doses of vaccine against yellow fever to achieve 80% vaccination coverage across countries by 2026 where the disease spreads among local populations, the rates of coverage will need to be much higher — as high as 95% in some areas, to eliminate local outbreaks of the disease, a study reported in the Journal of Infectious Diseases concludes.
Currently the disease is spread locally in sporadic outbreaks in regions across 34 African and 13 Central and South American countries, causing an estimated 200,000 cases of illness and as many as 30,000 deaths yearly. Launched in 2017 after an prolonged urban outbreak in Angola’s capital city that then spread to other countries, the Eliminate Yellow Fever Epidemics, or EYE strategy, led by WHO, with support from Gavi the Vaccine Alliance and Unicef aims to end local outbreaks of the disease by vaccinating those at greatest risk of exposure, prevent international spread, and contain outbreaks swiftly. The initiative aims to optimize the use of a vaccine that has been in short supply during large outbreaks, leading to fractionalized doses among local populations and leaving travelers without access to immunizations needed to enter some countries.
Researchers, led by Martial L. Ndeffo-Mbah of Texas A&M University’s School of Public Health analyzed the interaction of factors that included the impacts of local average temperatures, populations of aedes aegypti mosquitoes (the insect that spreads the yellow fever virus, as well as Zika, dengue and chikungunya), and social and economic factors driving dense urbanization that provides settings where the virus spreads rapidly in cycles from mosquitoes to humans.
They concluded that areas where combinations of these factors speed the spread of the disease, with increasing numbers of people put at risk from proximity to infected individuals by crowded settings and larger mosquito populations, at least 90%, and in some areas at least 95% of those potentially exposed would need to be vaccinated to quickly contain an outbreak. The authors recommend that targets for vaccine coverage in areas where the disease is endemic be tailored to local factors.
The article reporting the study, Global Risk and Elimination of Yellow Fever Epidemics is one of two examining questions on the most effective use of yellow fever vaccines, released in advance of an upcoming Journal of Infectious Diseases, with the other article revisiting the need for booster vaccinations following initial yellow fever vaccine administration. Together, a commentary also released in advance of the upcoming issue, the articles point to a potential need for a larger supply and accelerated production of the vaccines.