Yellow Fever

How yellow fever spreads

Yellow fever virus circulates in three cycles:

• Jungle (sylvatic) cycle: monkeys and forest mosquitoes keep the virus going. People are infected when working or traveling in forests.
• Intermediate (savannah) cycle: in African savannah areas, mosquitoes infect both monkeys and people in villages.
• Urban cycle: Aedes aegypti mosquitoes spread the virus from person to person in towns and cities, causing large epidemics.

The mosquitoes that carry it mostly bite during the day. Yellow fever does not spread through casual contact, coughing or touching. The WHO estimates tens of thousands of deaths a year, about 90% of them in Africa. Outbreaks in Brazil and Angola in the last decade spread to large cities.

Yellow fever symptoms

Acute phase

Symptoms start suddenly 3 to 6 days after the bite:

• fever and chills
• severe headache
• back pain and body aches
• nausea, vomiting and loss of appetite
• tiredness
• a relatively slow pulse for the height of the fever (Faget sign)
• red eyes and flushed face

Most people improve after 3 to 4 days and recover fully.

Toxic phase

About 15% of people who have symptoms improve for a few hours to a day, then become seriously ill again:

• high fever returns
• jaundice: yellowing of the skin and whites of the eyes from liver damage
• abdominal pain and vomiting, sometimes with blood or "black vomit"
• bleeding from the gums, nose, eyes or stomach, and easy bruising
• reduced urine from kidney failure
• confusion, seizures, shock

Between 30% and 60% of people who reach the toxic phase die, usually within 7 to 10 days.

Yellow fever vs other tropical fevers

How yellow fever is diagnosed

Diagnosis is based on symptoms and travel history and confirmed in specialist laboratories:

• PCR tests detect the virus in blood during the first few days.
• Antibody (IgM) tests become positive later. They can cross-react with dengue, Zika and recent yellow fever vaccination, so results need expert interpretation.
• Blood tests show liver damage (very high liver enzymes and bilirubin), low platelets, clotting problems and kidney failure in severe cases.

In the US, yellow fever must be reported to public health authorities.

Treatment

There is no specific antiviral treatment. Care is supportive, often in intensive care for severe cases:

• fluids and correction of salts
• treatment of low blood pressure and kidney failure, including dialysis
• blood products for bleeding
• oxygen and breathing support
• acetaminophen (paracetamol) for fever and pain, following the label dose, though doses may be limited in liver damage

Avoid aspirin and NSAIDs such as ibuprofen and naproxen, because they increase the risk of bleeding.

Patients should be protected from mosquitoes while sick to avoid spreading the virus locally. People who survive usually recover fully and are immune for life, though fatigue can last weeks.

Yellow fever vaccine

The live, weakened 17D vaccine is safe and highly effective.

• Who should get it: people aged 9 months or older traveling to or living in areas with risk of yellow fever.
• Timing: at least 10 days before arrival, when protection begins.
• Duration: a single dose protects most people for life. A booster is recommended for some groups, such as people who were vaccinated while pregnant, after a stem cell transplant, or with HIV, and for some lab workers and long-term travelers to high-risk areas.
• Certificate: the International Certificate of Vaccination or Prophylaxis (the "yellow card") is valid for life and is required for entry by some countries, especially for travelers arriving from countries with yellow fever risk. Check current requirements for every country on your itinerary.
• Where to get it: only at registered yellow fever vaccination centers.

Who should not have the vaccine

Because it is a live vaccine, it is usually not given to:

• babies under 6 months (and generally avoided between 6 and 8 months)
• people with severely weakened immunity, including symptomatic HIV, some cancer treatments and immunosuppressive drugs
• people with a thymus disorder or who have had their thymus removed
• people with a severe allergy to eggs, chicken proteins or gelatin

Extra caution applies to people 60 and older, who have a higher risk of rare serious reactions, and during pregnancy and breastfeeding. If you cannot have the vaccine, your provider can issue a medical waiver letter.

Side effects

Mild reactions such as low fever, headache and muscle aches occur in some people for a few days. Rare serious reactions include severe allergy, inflammation of the brain or nerves (YEL-AND), and a yellow fever-like illness affecting multiple organs (YEL-AVD), which is very rare and mostly affects older first-time recipients.

Preventing mosquito bites

• Use EPA-registered repellents with DEET, picaridin, IR3535 or oil of lemon eucalyptus.
• Wear long sleeves and trousers, and treat clothing with permethrin.
• Sleep in screened or air-conditioned rooms, or under a bed net.
• Remember that the mosquitoes that spread yellow fever bite during the day.

Frequently asked questions

What are the first symptoms of yellow fever?

The first symptoms appear 3 to 6 days after a mosquito bite: sudden fever, chills, severe headache, back pain, muscle aches, nausea and vomiting. Most people recover in 3 to 4 days, but some develop jaundice and bleeding after a brief improvement.

How long does the yellow fever vaccine last?

For most people, a single dose gives lifelong protection, and the vaccination certificate is valid for life. Some people, such as those vaccinated during pregnancy, after a stem cell transplant or with HIV, may need a booster. Ask a travel clinic about your situation.

Which countries have yellow fever?

Yellow fever occurs in parts of sub-Saharan Africa and tropical Central and South America, including countries such as Nigeria, the Democratic Republic of the Congo, Angola, Brazil, Peru, Colombia and Bolivia. Risk areas change, so check current CDC or WHO maps before traveling.

Is yellow fever contagious between people?

No. Yellow fever does not spread through direct contact, coughing or sharing items. It spreads only through mosquito bites. An infected person can, however, pass the virus to mosquitoes that bite them, which can then infect others in areas where those mosquitoes live.

Is there a cure for yellow fever?

There is no antiviral medicine that cures yellow fever. Treatment focuses on supporting the body with fluids, blood products, dialysis and intensive care. Vaccination is the most effective way to prevent the disease.