Yellow fever is an acute viral infectious disease transmitted to humans through the bite of infected mosquitoes. Though many cases of yellow fever are mild and self-limiting, yellow fever can also be a life-threatening disease causing haemorrhagic fever. Humans and monkeys are the principal animals to be infected. The virus is carried from one animal to another by a biting mosquito. The mosquito can also pass the virus via infected eggs to its offspring. The eggs produced are resistant to drying and lie dormant through dry conditions, hatching when the rainy season begins. Therefore, the mosquito is the true reservoir of the virus, ensuring transmission from one year to the next.
This viral disease occurs in tropical areas of Africa and South America, and each year there are an estimated 200,000 cases of yellow fever worldwide, leading to approximately 30,000 deaths. However, due to underreporting, only a small percentage of these cases are identified. Small numbers of imported cases also occur in countries free of yellow fever. Although yellow fever has never been reported from Asia, this region is at risk because the appropriate primates and mosquitoes are present.
33 countries, with a combined population of 508 million, are at risk in Africa. In the Americas, yellow fever is endemic in nine South American countries and in several Caribbean islands. Bolivia, Brazil, Colombia, Ecuador and Peru are considered at greatest risk.
Yellow fever is a hemorrhagic viral disease with a high case fatality rate. It is considered a reemerging infectious disease of remarkable importance. During the last outbreaks in Brazil (2016-2017), many cases of Yellow fever emerged despite highvaccination coverage in some areas. However, there are many areas and populations worldwide where vaccination coverage has been low for years (e.g. Nigeria), which increases the risk of major epidemics in such areas, as would be the case in many of the American territories. Several factors, including the vast border and migratory status of Brazil, the widespread distribution of Aedes mosquitoes and the lack of efficient health policies and surveillance systems, favor this complex epidemiological scenario of reemergence. Therefore, mass vaccination of the population at risk, public health awareness and preparedness are urgently needed in this region. This opinion article describes the current global epidemiological situation of Yellow fever, focusing especially on the Americas, as well the risk and vulnerabilities in the region that would be of concern for major expansion to other countries apart from Brazil. Also, imported risk from endemic area outside of Americas (i.e. Africa) are of current concern.