Vaccination remains the most important tool to prevent outbreaks of yellow fever. Here, an electron micrograph of Yellow Fever Virus virions.

Four out of ten infected travelers have died

An outbreak of yellow fever among monkeys has turned for the worse and spread, infecting ten travelers, according to the US Centers for Disease Control and Prevention (CDC). None of the ten travelers — who came from France, Romania, Germany, Switzerland, Chile, Argentina, and the Netherlands — had been vaccinated against yellow fever before they traveled. Sadly, four out of the ten travelers have died.

The yellow fever virus

The yellow fever virus, like the related Zika and dengue viruses, is spread by Aedes aegypti mosquitoes (that spread dengue and Zika) and Haemagogus mosquitoes that live in the jungle. The virus spreads typically in what is known as a jungle or sylvatic cycle, with transmission between mosquitoes and monkeys. Occasionally a person becomes infected, but human cases are rare in the jungle cycle.

Yellow fever is more lethal than its cousins, with a significant fatality rate. Case fatality rates (or the proportion of cases which are fatal) for reported cases are in the order of 15 to 50%. Since July 2017, the World Health Organization (WHO) and Pan American Health Organization (PAHO) have recorded over 400 yellow fever cases in Brazil, and 118 people have died. Yellow fever follows a seasonal pattern in Brazil, occurring mainly during the summer months. Brazil’s yellow fever outbreak has been spreading since 2016 and infiltrating areas where health experts initially thought to be free of yellow fever.

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Treatment or prevention?

There is no cure for yellow fever, but there is a vaccine. Last week, federal health officials strengthened their warnings about travel to Brazil, saying that anyone planning to travel there needs to get a yellow fever vaccine. Brazil received a record 6.6 million foreign tourists in 2016, boosted by visitors attending the Summer Olympic and Paralympic Games. The city of Rio de Janeiro alone received more than 1.1 million tourists during the 2017 Carnival.

The yellow fever vaccine

Yellow fever vaccination is recommended for all eligible persons aged ≥9 months, traveling to several areas in Brazil, including the states of São Paulo and Rio de Janeiro (especially Ilha Grande). Travelers who have not been vaccinated should avoid going to areas where vaccination is recommended. Those who are planning to visit areas in Brazil where yellow fever transmission is happening need to get vaccinated at least ten days before travel and follow recommendations for avoiding mosquito bites.

The Yellow fever vaccine is a live, weakened virus. It is given as a single shot. For people who remain at risk, a booster dose is recommended every ten years. Yellow fever vaccine may be given at the same time as most other vaccines. The vaccine provides effective immunity within 30 days for 99% of persons vaccinated. Nevertheless, there is a vaccine supply issue that most often coincides with the outbreaks: There are only four WHO-certified yellow fever vaccine manufacturers. Because yellow fever had been controlled for several decades before this recent resurgence, the demand for a vaccine has been low and vaccine manufacturers have not had the incentive to produce a sufficient amount of vaccines.

For more information on yellow fever, click this link to the US CDC Yellow Fever page.