What Is Yellow Fever?
Yellow fever is a serious and potentially fatal viral disease transmitted by infected Aedes and Haemagogus mosquitoes. The virus is endemic to tropical and subtropical regions of Africa and South America, where it circulates among mosquitoes and primates in forested areas. The disease gets its name from the jaundice that affects some patients, causing yellowing of the skin and eyes. While most infections are mild or asymptomatic, severe cases can progress to hemorrhagic fever with organ failure, carrying a fatality rate of 20% to 60%.
Symptoms and Disease Progression
Yellow fever symptoms typically appear 3 to 6 days after being bitten by an infected mosquito. The illness progresses through distinct phases, and recognizing early symptoms is critical for seeking timely medical care.
Initial Phase (Days 1-3)
- Sudden onset of fever, often reaching 39-40 degrees Celsius
- Severe headache and muscle pain, particularly in the back and knees
- Nausea, vomiting, and loss of appetite
- Dizziness and facial flushing
- Sensitivity to light
Toxic Phase (Approximately 15% of Cases)
Most patients improve after the initial phase, but approximately 15% enter a more severe toxic phase within 24 hours of apparent recovery. During this phase, high fever returns, jaundice develops as the liver is damaged, abdominal pain with vomiting occurs, bleeding can start from the mouth, nose, eyes, or stomach, and kidney function may deteriorate rapidly. The toxic phase requires immediate emergency medical treatment and is associated with the highest mortality rates.
Vaccination: Your Most Important Protection
The yellow fever vaccine (17D) is one of the most effective vaccines ever developed, providing immunity in 99% of recipients within 30 days. In 2016, the World Health Organization confirmed that a single dose provides lifelong protection, eliminating the need for booster shots every 10 years as was previously required. The vaccine must be administered at a designated Yellow Fever Vaccination Centre, and the International Certificate of Vaccination or Prophylaxis (ICVP) must be stamped and signed by the administering clinician.
Key Vaccination Facts
- Minimum age: 9 months for routine vaccination, 6 months during outbreaks
- Timing: Must be given at least 10 days before entering an endemic area for the certificate to be valid
- Validity: Lifetime, under WHO International Health Regulations amended in 2016
- Not recommended for: Pregnant women, individuals with severe egg allergies, people with compromised immune systems, and adults over 60 who have never been vaccinated (assess risk vs. benefit with a doctor)
- Side effects: Generally mild, including low-grade fever, headache, and muscle pain for a few days; serious adverse events are extremely rare
High-Risk Regions and Entry Requirements
Many countries require proof of yellow fever vaccination as a condition of entry, particularly for travelers arriving from or transiting through endemic areas. Failing to present a valid certificate can result in denied entry, quarantine, or vaccination at the port of arrival.
- Sub-Saharan Africa: The highest risk zone, with endemic transmission across West, Central, and East Africa including Nigeria, Democratic Republic of Congo, Angola, Ghana, and Cameroon
- South America: Tropical and subtropical regions including the Amazon basin, parts of Brazil, Colombia, Ecuador, Peru, Bolivia, and Venezuela
- Bolivia: Eastern departments below 2,300 meters (7,550 feet) elevation including Santa Cruz, Beni, Pando, and Cochabamba
- Countries with no yellow fever transmission that still require proof of vaccination for travelers arriving from endemic areas, including India, Thailand, South Africa, and many island nations
Additional Prevention Measures
While vaccination is the cornerstone of yellow fever prevention, travelers to endemic areas should also take steps to avoid mosquito bites. Unlike malaria-carrying Anopheles mosquitoes, the Aedes mosquitoes that transmit yellow fever are most active during daylight hours, particularly in the early morning and late afternoon.
- Apply insect repellent containing DEET, picaridin, or IR3535 to exposed skin throughout the day
- Wear light-colored, long-sleeved clothing that covers arms and legs
- Stay in accommodations with window screens or air conditioning
- Use mosquito nets if sleeping outdoors or in unscreened rooms, particularly in rural areas
- Avoid heavily forested or jungle areas where sylvatic (forest) transmission cycles occur between mosquitoes and monkeys
- Be especially cautious in rural areas below 2,300 meters elevation where risk is highest
What to Do If You Develop Symptoms
There is no specific antiviral treatment for yellow fever. Medical care focuses on managing symptoms, maintaining hydration, and supporting organ function. If you develop fever, headache, muscle pain, or any signs of jaundice during or after travel to an endemic area, seek medical attention immediately and inform your healthcare provider of your travel history and vaccination status. Avoid taking aspirin or non-steroidal anti-inflammatory drugs, as these can increase the risk of bleeding. Early supportive care significantly improves outcomes, so do not delay seeking treatment.
Post-Recovery Considerations
Survivors of yellow fever develop lifelong immunity to the virus and do not need subsequent vaccination. However, recovery from severe yellow fever can take weeks to months, and some patients experience prolonged fatigue, liver damage, or kidney complications that require ongoing medical monitoring. If you have recovered from yellow fever, carry documentation of your illness as it may exempt you from vaccination requirements for future travel. Inform any future healthcare providers of your history, as it can affect decisions about medications that are processed by the liver.
The International Certificate of Vaccination
The International Certificate of Vaccination or Prophylaxis (ICVP), commonly known as the yellow card due to its distinctive color, is a critical travel document for anyone visiting or transiting through yellow fever endemic areas. This standardized document, governed by the World Health Organization's International Health Regulations, serves as official proof that the holder has been vaccinated against yellow fever. It must be presented at immigration checkpoints in countries that require proof of vaccination, and failure to produce a valid certificate can result in denied entry, forced vaccination at the border, or quarantine for up to six days.
Keeping Your Yellow Card Safe
- Store the original certificate with your passport, as you may need to present it at immigration alongside your travel documents
- Make photocopies and take clear photographs of the certificate to store digitally as a backup, though only the original is officially accepted
- If your certificate is lost or damaged, contact the vaccination center that administered the vaccine, as they typically maintain records and can issue a replacement
- Keep the certificate even after it expires or is no longer needed for a specific trip, as some countries may request your full vaccination history during future visa applications
- If you receive a medical waiver exempting you from vaccination, carry the official waiver letter with you, as it must be presented in place of the certificate