Yellow Fever

Yellow fever (YF) is a mosquito-borne, viral haemorrhagic fever that is endemic in tropical regions of Africa and South America. "Aedes Aegypti" is the vector of YF virus in the urban human-to-human cycle of the transmission, whereas in the jungle (forest, sylvatic) monkey-to-monkey - and accidentally monkey-to-human - cycle, several different mosquito species are involved. About 90% of an estimated 200 000 annual cases of YF occur in Africa, where outbreaks are common and where both the urban and the jungle type of transmission operate. In South America, the jungle type of YF predominates, either in individual cases or localized outbreaks.

There is no specific antiviral treatment for the YF virus. A highly efficacious, live attenuated vaccine (17D) has been available for 60 years. One month following immunization, up to 99% of vaccinees show protective levels of neutralizing antibodies, and the immunity is likely to last for decades. Adverse events following YF vaccination are usually minor, although hypersensitivity to vaccine component may occasionally occur, and very rare cases of viral encephalitis or multiple organ failures have been reported. The rare adverse events should not deter the appropriate use of this highly valuable vaccine. In countries at risk for YF, this vaccine is recommended for individual and outbreak prevention, as well as outbreak control. The vaccine is also widely used for the protection of travellers to YF-endemic areas. Although there is no current shortage of YF vaccine at the global level, supplies may not be sufficient in the event of multiple large outbreaks in urban centres.

In countries at risk for YF, the use of the 17D vaccine is the main strategy recommended to rapidly build up YF immunity in the population at large. This prevention strategy has two components. The first component is the inclusion of the 17D vaccine in national childhood immunization programmes. For convenience and improved coverage, the YF vaccine should be administered simultaneously with the measles vaccine at approximately 9-12 months of age, but in a separate syringe and at a different injection site.

The second component is the implementation of mass preventive vaccination campaigns to protect susceptible older age groups. In the event of lilmited resources, assessment of the degree of risk can help prioritize areas for mass preventive campaigns.