Yellow fever (YF) is an acute viral haemorrhagic disease, caused by an arbovirus of the genus Flavivirus and family Flaviviridae. It is transmitted by the bite of female mosquitoes (several species of the Haemogogus and Aedes including Aedes aegypti). Primates and several species of mosquitoes are the only hosts. YF is endemic in the sylvatic setting in sub-Saharan Africa and the tropical regions of South America, but not in Asia. In the jungle form, YF is spread from mosquitoes to monkeys and also to humans (zoonosis). Urban YF may cause large outbreaks in cities and suburbs, in which infected mosquitoes transmit the virus to human. Outbreaks occur periodically in Africa and sporadically in South America. The presence of Aedes mosquito vector in Asia may contribute to its potential occurrence. Malaysia is vulnerable to YF transmission.

The incubation period is 2-5 days. Some cases are asymptomatic but most lead to an acute illness characterised by two phases. The infection is characterised by flu-like symptoms, including fever, headache, muscle pain, backache, anorexia, nausea, and vomiting, often with bradycardia. About 15% will develop a second phase with severe symptoms within 48 hours, including vomiting, abdominal pain, jaundice, haemorrhage, shock, and multiple organ failure. Haemorrhagic manifestations include epistaxis, gingival bleeding, haematemesis, melaena, and liver and renal failure. The acute form of the disease is viral haemorrhagic fever which can lead to death within 10-14 days in 50% of cases in non-indigenous individuals (namely travelers) and during epidemic. Among the indigenous populations in endemic areas, fatality is around 5%. There is no specific treatment for yellow fever. Patients are advised plenty of rest, fluids and regular painkillers (e.g. paracetamol or ibuprofen) for symptomatic relief of fever or aches.

WHO recommends that all endemic countries should introduce YF vaccine into their routine immunisation programmes. Preventive mass vaccination campaigns are recommended for inhabitants of areas at risk of YF where there is low vaccination coverage. Vaccination is recommended to be given to everyone aged ≥ 9 months, in any area with reported cases. Reported cases of human disease is the principal indicator of disease risk, but may be absent due to a high level of immunity in population, or not detected due to lack of surveillance. Travelers to endemic countries with risk of YF transmission of Africa, Central and South America are at risk. Some countries require proof of YF vaccination for entry. When traveling to low risk YF areas, travelers should take adequate measures to prevent mosquito bites during the daytime.

There are 3 reasons for YF vaccination:

• To provide individual protection and reduces the risks of infection for those living in epidemic and endemic YF areas.
• To protect individual travelers who may be exposed to YF infection.
• To prevent the spread of YF by viraemic travelers on an international scale.

This is to protect countries from the risk of importing or further spreading YF virus by establishing entry requirements on YF vaccination for travelers. The countries that require proof of vaccination are those where the disease may or may not occur, and where the mosquito vector and potential non-human primate hosts of YF are present.

Following an update by the World Health Assembly in 2014, WHO announced that as of 11 July 2016, existing and new YF vaccination certificates are valid for life starting 10 days after vaccination. Countries can no longer require travelers to show proof of re-vaccination or a booster dose as a condition of entry.