EXPLAINER: All you need to know about Yellow Fever

Yellow Fever is an acute viral disease transmitted by infected mosquitoes of the Aedes species.

Symptoms of this disease include fever, headache, jaundice,  muscle pain, nausea, vomiting, and fatigue.

According to a brief from the World Health Organization, a small proportion of patients who contract the virus develop severe symptoms and approximately half die within seven to 10 days.

Large epidemics of Yellow Fever occur when infected people introduce the virus into heavily populated areas with high mosquito density and where most people have little or no immunity, due to lack of vaccination.

"In these conditions, infected mosquitoes transmit the virus from person to person," WHO says.

Though there is currently no specific anti-viral drug for yellow fever, the disease is preventable through an effective vaccine, which is safe and affordable.

 A single dose of the vaccine is sufficient to confer sustained immunity and life-long protection against the disease.

"The vaccine provides effective immunity within 10 days for 80-100 per cent of the people vaccinated, and within 30 days for more than 99 per cent of people vaccinated," according to WHO.

Good supportive treatment in hospitals improves survival rates. Control of the vector and protection against their bites is an effective way of controlling yellow fever.

WHO says the yellow fever vaccine is safe, affordable and a single dose provides life-long protection against yellow fever disease. A booster dose of the vaccine is not needed.

Several vaccination strategies are used to prevent yellow fever disease and transmission: routine infant immunisation; mass vaccination campaigns designed to increase coverage in countries at risk; and vaccination of travellers going to yellow fever endemic areas.

In high-risk areas where vaccination coverage is low, prompt recognition and control of outbreaks using mass immunisation is critical. 

There have been rare reports of serious side-effects from the yellow fever vaccine. The rates for these severe 'adverse events following immunization' (AEFI), when the vaccine provokes an attack on the liver, the kidneys or on the nervous system are between 0 and 0.21 cases per 10 000 doses in regions where yellow fever is endemic, and from 0.09 to 0.4 cases per 10 000 doses in populations not exposed to the virus .

"The risk of AEFI is higher for people over 60 years of age and anyone with severe immunodeficiency due to symptomatic HIV/Aids or other causes, or who have a thymus disorder," WHO explains.

People over 60 years of age should be given the vaccine after a careful risk-benefit assessment.

People who are usually excluded from vaccination include: infants aged less than nine months; pregnant women — except during a yellow fever outbreak when the risk of infection is high; and people with severe allergies to egg protein.

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