Is There a Risk of Yellow Fever Virus Transmission in South Asian Countries with Hyperendemic Dengue?

The fact that yellow fever (YF) has never occurred in Asia remains an "unsolved mystery" in global health. Most countries in Asia with high Aedes aegypti mosquito density are considered "receptive" for YF transmission. Recently, health officials in Sri Lanka issued a public health alert on the potential spread of YF from a migrant group from West Africa. We performed an extensive review of literature pertaining to the risk of YF in Sri Lanka/South Asian region to understand the probability of actual risk and assist health authorities to form evidence informed public health policies/practices. Published data from epidemiological, historical, biological, molecular, and mathematical models were harnessed to assess the risk of YF in Asia. Using this data we examine a number of theories proposed to explain lack of YF in Asia. Considering the evidence available, we conclude that the probable risk of local transmission of YF is extremely low in Sri Lanka and for other South Asian countries despite a high Aedes aegypti density and associated dengue burden. This does not however exclude the future possibility of transmission in Asia, especially considering the rapid influx travelers from endemic areas, as we report, arriving in Sri Lanka.