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Yellow fever: Ending epidemics may require 95% vaccination coverage

Vaccination coverage of up to 95 percent might be required to eliminate yellow fever epidemics in high-risk transmission areas in Africa, a new study from Texas A&M’s College of Veterinary Medicine & Biomedical Sciences faculty-researcher Martial Ndeffo has found. In other areas of West Africa and the Equatorial region of Latin America, a vaccination coverage of 90 percent may be required to achieve elimination.

Vaccination coverage refers to the estimated percentage of individuals in a given area who have been immunized against a disease. Currently, the World Health Organization (WHO) recommends vaccination coverage of 60-80 percent to protect at-risk populations from yellow fever.

This vaccination threshold is defined in the WHO’s 2016 Eliminate Yellow Fever Epidemics (EYE) strategy, which aims to eliminate yellow fever outbreaks by 2026. In addition to achieving 60- 80 percent population-level immunity through vaccination, the strategy also includes preventing international spread of yellow fever and containing outbreaks rapidly.

“Our research shows that the current WHO recommendation is suboptimal for achieving global elimination of yellow fever epidemics,” Ndeffo said. “Though it may be sufficient to eliminate yellow fever outbreaks in a few countries, most countries will require higher vaccination coverage, at least in some regions of those countries.”

Ndeffo is an assistant professor in the Department of Veterinary Integrative Biosciences.

Yellow fever widely affects communities in tropical areas of Africa and Latin America. It is estimated to cause between 29,000 and 60,000 deaths annually. Recent large-scale outbreaks in Brazil, Angola, and the Democratic Republic of Congo have stressed the need for yellow fever elimination.

“Some countries are well on their way to achieve the required vaccination coverage, while some countries are lagging behind and will surely need substantial assistance from the international community,” Ndeffo said.

Ndeffo developed a model to predict the spread of yellow fever that takes into account multiple contributing factors.  From this model, they were able to estimate the vaccination threshold required for elimination of yellow fever in different regions.

“Several factors contribute to the variation in yellow fever transmission risk between and within countries. These include mosquito density, local temperature, and human exposure to mosquito (driven by socio-economic factors), among others,” Ndeffo said. “Our research clearly shows that optimal vaccination strategies should account for local heterogeneity (variation between communities) in yellow fever transmission risk.”

The study, published in The Journal of Infectious Diseases, provides possible insight into how public health officials should approach the eradication of other infectious diseases.

“Similar studies are needed for other diseases targeted for global elimination, as they will provide new insights into control efforts needed to achieve disease elimination at different geographical scales,” Ndeffo said.

This study also serves as a reminder to the unaffected world that yellow fever is still a major issue.

“Though much progress has been made in the global fight against yellow fever, much more needs to be done to achieve global elimination,” Ndeffo said. “Yellow fever remains a major global health problem and vaccination recommendations for those traveling to endemic countries should be taken very seriously.”