Yellow Fever
A mosquito-borne viral disease found in parts of Africa and South America.
What causes Yellow Fever?
Yellow fever is caused by yellow fever virus and spreads through bites from infected mosquitoes in parts of Africa and South America.
Symptoms and what to notice
Fever, headache, body aches, nausea, or vomiting; severe illness can cause jaundice, bleeding, shock, and organ failure.
How long can it last?
Symptoms often begin three to six days after infection; severe disease can progress rapidly.
How serious is Yellow Fever?
Severe yellow fever has a high fatality rate and needs urgent hospital care.
Treatment and management
No specific antiviral treatment is available; care is supportive. Aspirin and similar medicines may increase bleeding risk.
Vaccine: who and when?
Product: YF-VAX, a single-dose live vaccine for eligible people age 9 months and older. It should be given at least 10 days before travel; one dose protects most people for life, although selected travelers may need a booster or additional documentation based on destination and individual risk.
Cost note: Current U.S. travel-clinic self-pay examples are about $295 for one YF-VAX dose before any consultation or certificate fee. Prices and insurance coverage vary.
Trusted source
Patient cost varies by insurance, provider, location, and eligibility. Coverage and public-health access differ, so listed prices are examples rather than guaranteed patient prices. This page is for general education and does not replace individualized medical advice, diagnosis, vaccination assessment, or treatment.
Yellow Fever: History, Symptoms, Transmission, Treatment, Prevention, and Vaccine Information
Yellow fever is a potentially serious mosquito-borne viral disease found mainly in tropical and subtropical regions of Africa and South America. It is caused by yellow fever virus and is transmitted primarily through the bites of infected mosquitoes.
Many people infected with yellow fever virus develop no symptoms or only a relatively mild illness. When symptoms do occur, they commonly begin with fever, chills, severe headache, muscle and back pain, nausea, vomiting, fatigue, and weakness.
A smaller proportion of infected people develop a dangerous second phase of illness involving the liver, kidneys, cardiovascular system, and blood-clotting mechanisms. This severe form can cause jaundice, bleeding, shock, organ failure, and death.
The disease gets its name from jaundice, the yellow discoloration of the skin and whites of the eyes that can develop when severe infection damages the liver.
Yellow fever is particularly important because there is no specific antiviral treatment, but it is highly preventable through vaccination. A single dose of yellow fever vaccine provides long-lasting protection for most people.
Although yellow fever is now uncommon in countries such as the United States, it remains a major public health concern in parts of Africa and South America, where outbreaks can occur when vaccination coverage is inadequate and infected mosquitoes circulate among susceptible populations.
Important: This article is intended for general education and does not replace professional medical advice, diagnosis, vaccination assessment, or treatment.
What Is Yellow Fever?
Yellow fever is an acute viral infection caused by yellow fever virus.
The virus belongs to the family Flaviviridae and is closely related to several other medically important mosquito-borne viruses, including:
- Dengue virus
- Zika virus
- West Nile virus
- Japanese encephalitis virus
Yellow fever virus contains single-stranded RNA enclosed within a protein shell and a lipid-containing viral envelope.
The virus normally circulates between mosquitoes and primates. Humans can become part of this transmission cycle when infected mosquitoes bite them.
Yellow fever does not ordinarily spread directly from one person to another through coughing, sneezing, casual touching, or ordinary social contact.
Yellow Fever Quick Facts
| Feature | Information |
|---|---|
| Disease | Yellow fever |
| Cause | Yellow fever virus |
| Virus family | Flaviviridae |
| Genetic material | Single-stranded RNA |
| Primary transmission | Bite of an infected mosquito |
| Important urban vector | Aedes aegypti |
| Other vectors | Haemagogus, Sabethes, and other mosquito species |
| Main regions | Tropical Africa and South America |
| Incubation period | Usually 3 to 6 days |
| Major severe complication | Liver and kidney injury with hemorrhage and shock |
| Characteristic severe sign | Jaundice |
| Specific antiviral treatment | None |
| Vaccine available | Yes |
| U.S. vaccine | YF-VAX |
| Routine U.S. childhood vaccine | No |
| Typical vaccine schedule | One dose for most eligible travelers |
| Main prevention | Vaccination and mosquito bite prevention |
The History of Yellow Fever
Yellow fever has played an unusually important role in medical history.
For centuries, epidemics killed thousands of people in Africa, the Americas, the Caribbean, and occasionally Europe. The disease influenced migration, warfare, international trade, urban development, public health policy, and even major engineering projects.
Yellow fever also helped establish one of the fundamental principles of modern infectious-disease science: insects can transmit human diseases.
1600s: Early Recognized Yellow Fever Epidemics
Diseases resembling yellow fever were described before the seventeenth century, but historical diagnosis is difficult because laboratory testing did not exist.
One of the earliest generally accepted descriptions of a yellow fever epidemic dates to 1648, when outbreaks occurred in the Yucatán region and the Caribbean.
During the following centuries, yellow fever became repeatedly associated with tropical ports, shipping routes, and areas connected by trade.
Historical and epidemiological evidence supports the view that yellow fever originated in Africa and was introduced into the Americas during the era of transatlantic trade and forced migration.
1700s and 1800s: Devastating Epidemics
Yellow fever became one of the most feared infectious diseases in the Americas.
Cities could experience explosive epidemics during warmer months when mosquito populations increased.
At the time, the cause was unknown.
Many physicians believed disease arose from miasma, meaning poisonous or contaminated air produced by swamps, sewage, decaying organic material, or unsanitary environments.
Because mosquitoes were not yet known to transmit infectious agents, mosquito control was not initially recognized as a central prevention strategy.
The Philadelphia Yellow Fever Epidemic of 1793
One of the most famous American outbreaks occurred in Philadelphia in 1793.
Philadelphia was then the temporary capital of the United States.
The epidemic killed approximately 5,000 people, representing a substantial portion of the city’s population.
Large numbers of residents fled the city.
Yellow fever epidemics continued to affect American cities during the nineteenth century, particularly communities connected to southern ports and river transportation.
CDC historical research estimates that at least 100,000 deaths in the United States were attributed to yellow fever between 1693 and 1905.
1853: New Orleans Experiences a Catastrophic Epidemic
Yellow fever was particularly feared in New Orleans.
During the city’s major 1853 epidemic, approximately 9,000 people died.
Another major epidemic in 1878 spread along the Mississippi River and caused an estimated 16,000 to 20,000 deaths from the Gulf Coast northward.
These outbreaks disrupted commerce, transportation, communities, and local economies.
1881: Carlos Finlay Proposes Mosquito Transmission
A major breakthrough came from Cuban physician Carlos Juan Finlay.
In 1881, Finlay proposed that a mosquito was responsible for carrying yellow fever from an infected person to a susceptible person.
His theory was revolutionary.
Most medical authorities at the time still focused on contaminated objects, direct contagion, or environmental causes.
Finlay conducted experiments attempting to demonstrate mosquito transmission, but the evidence initially failed to persuade much of the medical community.
Nevertheless, his central idea proved correct.
1900: The Walter Reed Commission Confirms Mosquito Transmission
During the Spanish-American War and subsequent U.S. military presence in Cuba, yellow fever posed a serious threat to soldiers.
The U.S. Army formed the Yellow Fever Commission, led by physician Walter Reed.
The team included:
- Walter Reed
- James Carroll
- Aristides Agramonte
- Jesse William Lazear
Building on Finlay’s mosquito theory and additional epidemiological observations, the commission conducted experiments in Cuba.
Their work established that yellow fever could be transmitted by mosquitoes and was not primarily spread through contaminated clothing, bedding, or direct contact with patients.
The mosquito involved was what is now known as Aedes aegypti.
The researchers also produced evidence that the infectious agent could pass through filters that trapped bacteria, supporting the conclusion that the causative organism was much smaller than known bacteria.
Yellow fever therefore became an important early disease in the emerging science of virology.
The Human Cost of the Research
Research into yellow fever was dangerous.
Some investigators and volunteers became infected during experiments.
Jesse Lazear, one of the members of the Yellow Fever Commission, contracted yellow fever and died in 1900.
The experiments also raise significant ethical questions when considered by modern research standards. They took place decades before today’s systems of institutional review boards, standardized informed consent, and modern protections for research participants.
Mosquito Control Transforms Yellow Fever Prevention
Once mosquito transmission was established, public health officials could target the vector instead of attempting to disinfect everything touched by infected patients.
William C. Gorgas led extensive mosquito-control campaigns in Havana.
Measures included:
- Eliminating standing water
- Destroying mosquito breeding sites
- Screening buildings
- Protecting yellow fever patients from mosquitoes
- Controlling adult mosquito populations
These measures dramatically reduced yellow fever transmission in Havana.
Similar strategies were later used during work on the Panama Canal, where mosquito-borne diseases had caused major problems for workers.
1905: The Last Major U.S. Yellow Fever Outbreak
The last major urban yellow fever epidemic in the United States occurred in New Orleans in 1905.
Mosquito-control campaigns eventually interrupted transmission.
Locally acquired yellow fever subsequently disappeared from the United States, although imported infections remain possible when travelers become infected abroad.
1927: Scientists Isolate Yellow Fever Virus
Understanding mosquito transmission was a major achievement, but scientists still needed to isolate and study the actual virus.
A breakthrough occurred in 1927.
Researchers isolated yellow fever virus from the blood of a West African man named Asibi, who had developed a relatively mild case of yellow fever in what is now Ghana.
The strain became known as the Asibi strain.
This isolate would later become extremely important because it provided the starting material from which the modern yellow fever vaccine was developed.
1930s: Max Theiler Works Toward a Vaccine
Scientist Max Theiler and his colleagues studied yellow fever virus at the Rockefeller Foundation.
Researchers repeatedly grew and passed the Asibi virus through laboratory systems.
Through repeated passage, the virus gradually became less capable of causing severe disease while remaining capable of stimulating an immune response.
The result was an attenuated strain called 17D.
1937: The 17D Yellow Fever Vaccine Is Developed
The 17D vaccine was developed in 1937.
It was a live attenuated vaccine, meaning it contained a weakened version of yellow fever virus capable of producing immunity without causing ordinary yellow fever disease in healthy vaccine recipients.
The vaccine proved highly effective.
WHO records indicate that yellow fever vaccination began expanding rapidly from the late 1930s onward.
Modern yellow fever vaccines remain descendants of the 17D vaccine strain.
1938: Mass Vaccination Begins
Large-scale use expanded rapidly.
Mass vaccination programs began in Brazil and later across parts of Africa.
Yellow fever incidence fell dramatically in populations where high vaccination coverage was achieved.
This demonstrated one of the earliest major successes of vaccination against a viral disease transmitted by mosquitoes.
1951: Max Theiler Receives the Nobel Prize
In 1951, Max Theiler received the Nobel Prize in Physiology or Medicine for his discoveries concerning yellow fever and its prevention.
His development of an effective yellow fever vaccine became one of the landmark achievements of twentieth-century medicine.
The vaccine developed from this research remains the foundation of yellow fever vaccination today.
How Does Yellow Fever Virus Infect the Body?
After an infected mosquito bites a person, virus contained in mosquito saliva enters the skin.
The virus begins multiplying locally and can reach nearby lymph nodes.
It subsequently enters the bloodstream, producing viremia, meaning virus is circulating in the blood.
Yellow fever virus can then reach multiple organs.
The liver is particularly important.
In severe disease, viral infection and the body’s immune response can cause extensive liver injury.
Other organs that can be affected include:
- Kidneys
- Heart
- Spleen
- Blood vessels
- Bone marrow
- Brain in some cases
Liver damage interferes with normal metabolism and blood-clotting functions.
This contributes to jaundice and bleeding in severe yellow fever.
Kidney dysfunction and cardiovascular instability can develop as the illness progresses.
How Does Yellow Fever Spread?
Yellow fever is primarily transmitted by infected mosquitoes.
Mosquitoes acquire yellow fever virus after feeding on an infected human or non-human primate.
The virus multiplies inside the mosquito.
Once infectious, the mosquito can transmit the virus when it bites another susceptible primate or human.
Can Yellow Fever Spread Directly Between People?
Ordinary person-to-person transmission does not occur through:
- Coughing
- Sneezing
- Touching
- Sharing food
- Being in the same room
A person with yellow fever can, however, infect a mosquito when sufficient virus is circulating in the person’s blood.
That mosquito can later infect another person.
Rare blood-associated transmission is also biologically possible.
CDC notes that bloodborne, transplantation, and perinatal transmission have been documented or considered possible under particular circumstances.
What Are the Three Yellow Fever Transmission Cycles?
One of the distinguishing features of yellow fever epidemiology is that the virus can circulate through several ecological cycles.
There are three recognized transmission cycles:
- Jungle or sylvatic
- Intermediate or savannah
- Urban
Jungle or Sylvatic Yellow Fever
In forests, yellow fever virus circulates primarily between:
Monkeys → mosquitoes → monkeys
Humans can become accidental hosts when they enter forested areas and are bitten by infected mosquitoes.
In South America, important jungle vectors include mosquitoes belonging to the genera:
- Haemagogus
- Sabethes
In Africa, several Aedes species can participate in sylvatic transmission.
People whose work or activities take them into forest environments can therefore be exposed even when no urban outbreak is occurring.
Intermediate or Savannah Yellow Fever
Intermediate transmission occurs mainly in Africa.
Semi-domestic mosquitoes living around forest edges and villages can transmit the virus among both monkeys and humans.
This creates small outbreaks involving multiple villages or communities.
WHO identifies intermediate transmission as an important form of yellow fever spread in Africa.
Urban Yellow Fever
Urban yellow fever has the greatest potential to produce large epidemics.
An infected person enters a densely populated area containing:
- Large numbers of Aedes aegypti mosquitoes
- Many unvaccinated people
- Suitable mosquito breeding conditions
Urban mosquitoes bite the infected person, acquire yellow fever virus, and later transmit it to other humans.
The transmission cycle then becomes:
Human → mosquito → human
This cycle can produce explosive outbreaks if vaccination coverage is low.
How Long Is Someone With Yellow Fever Infectious to Mosquitoes?
People with yellow fever develop relatively high levels of virus in their blood during the early phase of illness.
CDC reports that infected people can be infectious to mosquitoes shortly before fever begins and for approximately the first five days after symptom onset.
For this reason, people with suspected yellow fever should be protected from mosquito bites during the early illness.
Preventing mosquitoes from biting an infected patient helps prevent those mosquitoes from transmitting the virus to additional people.
What Is the Incubation Period for Yellow Fever?
The incubation period is the interval between infection and the appearance of symptoms.
For symptomatic yellow fever, the incubation period is usually approximately 3 to 6 days after the bite of an infected mosquito.
A person typically does not become ill immediately after being bitten.
Do All People Infected With Yellow Fever Become Sick?
No.
Most yellow fever virus infections are asymptomatic or relatively mild.
Some people never know they were infected.
Among people who develop initial symptoms, most recover without progressing to severe yellow fever.
The danger lies in the smaller group that progresses to severe disease.
What Are the Early Symptoms of Yellow Fever?
Yellow fever often begins suddenly.
Initial symptoms can include:
- Fever
- Chills
- Severe headache
- Back pain
- General muscle aches
- Weakness
- Fatigue
- Nausea
- Vomiting
- Dizziness
- Loss of appetite
CDC notes that fever can reach approximately 104°F or 40°C in some symptomatic patients.
Most people improve after this initial phase.
What Is Faget Sign?
An interesting classical finding associated with yellow fever is Faget sign.
Normally, heart rate tends to increase when body temperature rises.
In some yellow fever patients, the pulse may remain unusually slow relative to the degree of fever.
This relative bradycardia is known as Faget sign.
It can occur with yellow fever but is not specific enough to diagnose the disease by itself.
What Happens During Severe Yellow Fever?
For approximately 1 in 7 people who develop initial symptoms, an apparent recovery may be followed by a dangerous recurrence of illness.
The person may feel better for several hours or approximately a day.
Then severe symptoms can return.
This advanced stage has historically been called the toxic phase.
Possible symptoms include:
- High fever
- Jaundice
- Severe nausea
- Repeated vomiting
- Abdominal or epigastric pain
- Kidney dysfunction
- Reduced urine output
- Cardiovascular instability
- Bleeding
- Shock
- Multiple-organ failure
Why Does Yellow Fever Cause Jaundice?
Jaundice occurs when bilirubin accumulates in the body.
Bilirubin is normally processed by the liver.
Severe yellow fever can substantially injure liver cells.
When the damaged liver cannot process bilirubin normally, bilirubin concentrations rise.
This can cause yellow discoloration of:
- Skin
- Whites of the eyes
- Mucous membranes
The characteristic yellow appearance gave the disease its modern name.
Can Yellow Fever Cause Bleeding?
Yes.
Severe yellow fever can disrupt normal blood clotting.
Possible bleeding manifestations include:
- Nosebleeds
- Bleeding gums
- Gastrointestinal bleeding
- Blood in urine
- Small skin hemorrhages
- Bruising
- Bleeding from injection or needle sites
CDC also describes hematemesis, meaning vomiting blood, and melena, meaning dark stools caused by gastrointestinal bleeding, among possible severe manifestations.
What Is “Black Vomit” in Yellow Fever?
Historical descriptions of yellow fever frequently mention black vomit, sometimes called vomito negro.
This does not mean the virus produces a black substance.
Instead, severe gastrointestinal bleeding can result in partially digested blood being vomited.
Blood exposed to stomach acid can become dark brown or black and may resemble coffee grounds.
Before modern intensive medical care, black vomit became one of the most feared signs of advanced yellow fever.
How Dangerous Is Severe Yellow Fever?
Severe yellow fever is a medical emergency.
CDC estimates that approximately 30% to 60% of people who develop severe yellow fever die.
Possible causes of death include:
- Severe liver failure
- Kidney failure
- Shock
- Major bleeding
- Cardiovascular collapse
- Multiple-organ failure
This high fatality rate is one of the main reasons vaccination is so important for people at genuine risk of exposure.
Who Is Most at Risk of Yellow Fever?
Risk depends heavily on geographic exposure and vaccination status.
Important risk groups include:
Unvaccinated People in Endemic Areas
People who live in areas where yellow fever circulates and who have not been vaccinated can be vulnerable during outbreaks.
Travelers
Unvaccinated travelers visiting risk areas in:
- Africa
- South America
may become infected through mosquito bites.
Risk varies considerably according to:
- Country
- Exact destination
- Season
- Length of travel
- Vaccination status
- Local outbreaks
- Outdoor activities
- Forest exposure
People Working in Forested Areas
People engaged in activities such as:
- Forestry
- Agriculture
- Mining
- Field research
- Military operations
- Ecotourism
may encounter mosquitoes involved in sylvatic transmission.
How Is Yellow Fever Diagnosed?
Yellow fever can be difficult to diagnose based on symptoms alone.
Early disease can resemble many other infections.
Important possible alternative diagnoses include:
- Dengue
- Malaria
- Leptospirosis
- Viral hepatitis
- Other viral hemorrhagic fevers
- Severe bacterial infections
A clinician evaluating possible yellow fever considers:
- Symptoms
- Travel history
- Geographic exposure
- Vaccination history
- Timing of travel
- Local outbreak information
- Laboratory test results
What Tests Are Used for Yellow Fever?
Different tests are useful at different stages of the illness.
RT-PCR and Other Molecular Tests
Early in the infection, yellow fever viral RNA may be detected using nucleic acid amplification methods such as:
RT-PCR
These tests are most useful during the early period when virus is still circulating in the bloodstream.
CDC notes that viral RNA is typically most detectable during approximately the first week of illness.
A negative PCR result does not always exclude yellow fever because viremia can be brief.
Antibody Testing
Later in the illness, diagnosis can rely on detection of antibodies against yellow fever virus.
Tests can include:
- Yellow fever IgM
- Neutralizing antibody testing
- Plaque reduction neutralization testing, or PRNT
Testing can be complicated by previous vaccination and by immune responses to related flaviviruses.
For example, antibodies against dengue or West Nile viruses can sometimes cross-react in serological tests.
More specific neutralization testing may therefore be required.
Is Yellow Fever a Reportable Disease in the United States?
Yes.
Yellow fever is a nationally notifiable disease in the United States.
Suspected cases involving relevant travel or exposure history should be evaluated in coordination with public health authorities.
Specialized laboratory testing may be coordinated through state or local health departments and CDC.
How Is Yellow Fever Treated?
There is currently no specific antiviral drug approved to cure yellow fever.
Treatment is supportive.
For mild cases, medical management may include:
- Rest
- Fluids
- Fever management
- Appropriate pain relief
- Clinical monitoring
Patients with serious symptoms should be hospitalized.
Severe cases may require:
- Intravenous fluids
- Oxygen
- Blood-pressure support
- Treatment of bleeding
- Kidney monitoring
- Dialysis for severe kidney failure
- Mechanical ventilation
- Intensive-care support
- Treatment of secondary complications
Because the disease can deteriorate rapidly, severe yellow fever should not be managed solely at home.
Why Should Aspirin and NSAIDs Be Avoided?
Yellow fever can cause bleeding abnormalities.
CDC advises patients with suspected yellow fever to avoid aspirin and other nonsteroidal anti-inflammatory drugs, or NSAIDs, because these medications can increase bleeding risk.
Examples include:
- Aspirin
- Ibuprofen
- Naproxen
Medication decisions should be made with a healthcare professional, particularly when the patient has other medical conditions or takes prescription medicines.
Does Recovery Produce Immunity?
People who recover from yellow fever are generally believed to develop long-lasting immunity against subsequent yellow fever infection.
CDC states that people who have been infected are likely to be protected against future infections.
Is There a Yellow Fever Vaccine?
Yes.
Unlike Zika virus disease, yellow fever has a highly effective vaccine.
Yellow fever vaccine is a live attenuated viral vaccine derived from the historic 17D vaccine lineage.
Vaccination remains the most important tool for preventing yellow fever disease and controlling outbreaks.
What Yellow Fever Vaccine Is Used in the United States?
The licensed yellow fever vaccine used in the United States is:
YF-VAX
It is a live attenuated yellow fever virus vaccine.
It is administered at designated yellow fever vaccination centers.
Yellow Fever Vaccine Quick Reference
| Question | Current Information |
|---|---|
| Does a vaccine exist? | Yes |
| U.S. vaccine | YF-VAX |
| Vaccine type | Live attenuated |
| Vaccine lineage | 17D |
| Routine U.S. childhood vaccine | No |
| Typical minimum recommended travel age | 9 months |
| Number of doses for most people | One |
| Protection | Long-lasting, generally lifelong for most people |
| International certificate available | Yes |
| Booster required for most travelers | No |
| Serious adverse events | Rare but possible |
Who Should Receive Yellow Fever Vaccine?
CDC recommends yellow fever vaccination for most people aged 9 months or older who are traveling to or living in areas of Africa or South America where yellow fever transmission poses a risk.
Some countries also require proof of vaccination as a condition of entry.
Vaccination decisions should account for:
- Destination
- Travel route
- Current outbreak activity
- Length of stay
- Age
- Health conditions
- Immune status
- Pregnancy
- Vaccine contraindications
- Entry requirements
How Effective Is Yellow Fever Vaccine?
Yellow fever vaccine produces a strong immune response.
WHO reports that approximately 80% to 100% of vaccinated people develop protective immunity within 10 days, with more than 99% developing protective antibodies within 30 days.
For most healthy people, protection following one dose is extremely long-lasting.
Is One Dose Enough?
For most people, one dose is sufficient.
CDC’s Advisory Committee on Immunization Practices states that one dose provides long-lasting protection and is adequate for most travelers.
WHO considers a single dose sufficient to provide lifelong protection for most people.
However, additional vaccination may be recommended or considered for certain groups.
These can include:
- Some people who were pregnant when first vaccinated
- People who subsequently underwent hematopoietic stem cell transplantation
- Some people living with HIV
- Certain travelers entering especially high-risk settings 10 or more years after vaccination
- Laboratory personnel routinely handling wild-type yellow fever virus
What Is the Yellow Card?
Travelers sometimes refer to the International Certificate of Vaccination or Prophylaxis, or ICVP, as the “yellow card.”
It provides internationally recognized documentation of yellow fever vaccination.
Some countries require this certificate from:
- All incoming travelers
- Travelers arriving from countries with yellow fever risk
- Travelers who have spent a defined period in a yellow fever risk area
- Travelers who transited through certain risk areas
Vaccination requirements can differ from CDC medical recommendations.
A country may require proof of vaccination primarily to prevent yellow fever from being imported into that country.
CDC notes that under current International Health Regulations, a completed certificate for yellow fever vaccination is generally valid for the lifetime of the vaccinated person.
Travelers should always check current requirements for their specific itinerary before departure.
Who Should Not Receive Yellow Fever Vaccine?
Because the vaccine contains live attenuated virus, it is not appropriate for everyone.
CDC lists several contraindications, including:
- Severe allergy to a vaccine component
- Age younger than 6 months
- Symptomatic HIV infection or severe immune suppression
- Certain thymus disorders associated with abnormal immune function
- Primary immunodeficiency
- Malignant neoplasms associated with immune suppression
- Organ transplantation
- Certain immunosuppressive or immunomodulatory treatments
Vaccination decisions for these individuals require professional assessment.
Who Requires Special Assessment Before Vaccination?
CDC identifies several situations as precautions, meaning vaccination is not automatically prohibited but requires careful consideration of risks and benefits.
These include:
- Infants aged 6 through 8 months
- Adults aged 60 years or older
- Pregnancy
- Breastfeeding
- Certain people with asymptomatic HIV infection
A clinician experienced in travel medicine can compare the risk of exposure with the person’s risk of an adverse vaccine reaction.
What Are Common Yellow Fever Vaccine Side Effects?
Most reactions are mild.
Possible reactions include:
- Headache
- Muscle aches
- Low-grade fever
- Injection-site discomfort
- Swelling
- Local tenderness
These symptoms generally resolve without serious consequences.
The FDA prescribing information notes that mild headache, myalgia, low-grade fever, and local reactions can occur following YF-VAX administration.
What Serious Vaccine Reactions Can Occur?
Serious adverse reactions are rare, but they are important because some can be life-threatening.
They include:
Anaphylaxis
A severe allergic reaction can occur, particularly in someone allergic to a vaccine component.
Yellow Fever Vaccine-Associated Neurologic Disease
Known as YEL-AND, this involves neurological complications following vaccination.
Yellow Fever Vaccine-Associated Viscerotropic Disease
Known as YEL-AVD, this extremely rare complication resembles severe wild-type yellow fever and can involve multiple organs.
These risks are why vaccination should be given to people who have a genuine exposure risk or an international requirement, after contraindications and precautions have been considered.
Why Is Age 60 or Older a Vaccine Precaution?
Serious systemic reactions to the live yellow fever vaccine occur more often in older adults than in younger vaccine recipients.
The FDA notes an increased risk of severe systemic adverse reactions among people aged 60 years and older.
This does not mean that everyone over 60 should avoid vaccination.
Instead, the clinician and traveler should consider:
- Actual yellow fever risk at the destination
- Length of exposure
- Current outbreak conditions
- Whether vaccination is required for entry
- Medical history
- Risks associated with vaccination
How Can Yellow Fever Be Prevented Besides Vaccination?
Vaccination is highly effective, but mosquito protection remains important.
Even vaccinated travelers need protection because mosquitoes in yellow fever areas can transmit other illnesses such as:
- Dengue
- Zika
- Chikungunya
- Malaria in areas where malaria-carrying mosquitoes are also present
Use Insect Repellent
Use an appropriate insect repellent according to label directions.
Wear Protective Clothing
Consider:
- Long-sleeved shirts
- Long pants
- Socks
- Closed shoes when practical
Treat Clothing and Gear
CDC recommends that travelers consider clothing and gear treated with 0.5% permethrin.
Permethrin should be used according to product instructions and should not be applied directly to the skin unless the specific product is labeled for that use.
Choose Protected Accommodation
Where possible, stay in accommodations with:
- Window screens
- Door screens
- Air conditioning
Mosquito nets can provide additional protection in appropriate settings.
Eliminate Standing Water
Aedes aegypti mosquitoes frequently breed in artificial water containers.
Potential breeding sites include:
- Buckets
- Flowerpots
- Tires
- Water-storage containers
- Bottles
- Gutters
- Outdoor containers
Removing or properly covering standing water can reduce mosquito breeding near homes.
Do Yellow Fever Mosquitoes Bite Only at Night?
No.
Important yellow fever mosquito vectors can bite during daylight hours.
CDC advises mosquito precautions during both day and night in areas where yellow fever is a risk.
This differs from the common misconception that mosquito protection is necessary only after sunset.
How Widespread Is Yellow Fever Today?
Yellow fever continues to circulate in tropical areas of Africa and South America.
WHO’s October 2025 assessment classified 27 African countries and 13 Latin American countries as high-risk for yellow fever outbreaks.
Disease burden is particularly high in Africa.
WHO estimated approximately 67,000 to 173,000 severe infections and 31,000 to 82,000 deaths annually across Africa and the Americas, with most of the burden occurring in Africa.
These are modeled estimates because many infections occur in locations where surveillance and laboratory confirmation are limited.
What Is the Yellow Fever Situation in 2026?
Yellow fever remains an active international public health concern.
In a WHO situation report dated June 24, 2026, yellow fever transmission was reported in both Africa and the Americas.
From January 1 through May 26, 2026, six countries in the Americas reported 79 human infections, together with multiple infections among non-human primates.
WHO also reported continuing yellow fever activity across high-risk African countries during the first months of 2026.
The geographic and numerical situation can change, so travelers should use current outbreak and travel information rather than assuming that historical risk maps remain accurate.
Why Can Yellow Fever Not Be Eradicated Easily?
Vaccination can prevent human disease, but completely eliminating yellow fever virus is extremely difficult because humans are not its only hosts.
Yellow fever virus has a natural sylvatic reservoir involving:
- Non-human primates
- Forest mosquitoes
Even if urban human transmission is stopped, virus can continue circulating within forest ecosystems.
Humans entering these environments can become infected.
That means eradication cannot be achieved simply by vaccinating humans or eliminating urban transmission.
Instead, public health programs focus on:
- High vaccination coverage
- Outbreak detection
- Mosquito control
- Surveillance
- Rapid emergency vaccination
- Preventing urban transmission
What Is the WHO EYE Strategy?
WHO and international partners developed the Eliminate Yellow Fever Epidemics, or EYE, Strategy in response to continued yellow fever outbreaks.
Its major objectives include:
- Protecting populations at risk
- Preventing international spread
- Containing outbreaks rapidly
The strategy emphasizes routine vaccination, preventive mass vaccination campaigns, surveillance, laboratory capacity, and rapid outbreak response.
Is Yellow Fever the Same as Dengue?
No.
Yellow fever and dengue are caused by different viruses, although both belong to the Flaviviridae family and can be transmitted by Aedes aegypti mosquitoes.
| Feature | Yellow Fever | Dengue |
|---|---|---|
| Virus | Yellow fever virus | Dengue virus |
| Main vector | Mosquitoes | Mosquitoes |
| Aedes aegypti involved | Yes | Yes |
| Jaundice | Important in severe disease | Less characteristic |
| Severe bleeding | Can occur | Can occur |
| Severe liver damage | Characteristic of severe yellow fever | Can occur but differs clinically |
| Approved vaccine | Yes | Yes, but indications differ |
| Routine antiviral cure | No | No |
| Geographic distribution | Mainly Africa and South America | Much broader tropical and subtropical distribution |
Laboratory testing may be required because early symptoms can overlap.
Is Yellow Fever Related to Zika?
Yes, virologically, but they are different diseases.
Yellow fever virus and Zika virus belong to the same broader virus family, Flaviviridae.
Both can be transmitted by Aedes mosquitoes.
However, their major medical complications differ.
Zika is especially important because infection during pregnancy can cause congenital abnormalities.
Yellow fever is especially dangerous because severe infection can cause:
- Liver failure
- Jaundice
- Hemorrhage
- Kidney failure
- Shock
- Death
Yellow fever also has an effective licensed vaccine, while there is currently no approved Zika vaccine for routine use.
Can Yellow Fever Return to Areas Where It Has Disappeared?
Potentially, yes.
The risk depends on several factors:
- Presence of mosquito species capable of transmission
- Introduction of yellow fever virus by an infected traveler
- Low population immunity
- Environmental conditions
- Mosquito abundance
- Public health response
This possibility is one reason countries monitor travelers and, in certain situations, require proof of yellow fever vaccination.
When Should Someone Seek Medical Care?
A person should contact a healthcare provider promptly if they develop fever or other compatible symptoms after recent travel to a yellow fever risk area.
The clinician should be told:
- Countries visited
- Exact locations if known
- Dates of travel
- Mosquito exposure
- Yellow fever vaccination status
- Date of vaccination
- Onset of symptoms
Seek Urgent Medical Care for Severe Symptoms
Emergency evaluation is particularly important for:
- Yellow skin or eyes
- Repeated vomiting
- Vomiting blood
- Blood in stool
- Unexplained bleeding
- Confusion
- Severe weakness
- Difficulty breathing
- Very low urine output
- Fainting
- Severe abdominal pain
- Shock-like symptoms
- Rapid deterioration
Severe yellow fever can progress to life-threatening liver, kidney, cardiovascular, and multi-organ failure.
Frequently Asked Questions About Yellow Fever
Why is it called yellow fever?
The word “yellow” refers to jaundice, which can make the skin and whites of the eyes appear yellow when severe infection damages the liver.
Is yellow fever caused by bacteria?
No.
Yellow fever is caused by a virus.
Antibiotics therefore do not cure yellow fever.
How is yellow fever transmitted?
The primary route is the bite of an infected mosquito.
Can yellow fever spread from one person to another by touching?
No.
Yellow fever is not normally spread through casual contact, coughing, sneezing, or touching.
What is the incubation period?
Symptoms usually develop approximately 3 to 6 days after infection.
Does everyone infected with yellow fever become seriously ill?
No.
Most infections are asymptomatic or mild.
Only a smaller proportion progresses to severe yellow fever.
What percentage of symptomatic patients develop severe disease?
CDC currently describes severe recurrence in approximately 1 out of 7 people who develop initial symptoms.
How deadly is severe yellow fever?
Approximately 30% to 60% of people who develop severe yellow fever may die.
Can yellow fever damage the liver?
Yes.
Liver injury is one of the defining complications of severe yellow fever and can result in jaundice and abnormal blood clotting.
Can yellow fever damage the kidneys?
Yes.
Severe disease can cause renal insufficiency and, in extreme cases, kidney failure.
Is there a cure for yellow fever?
There is no specific antiviral cure currently available.
Treatment is supportive.
Is there a yellow fever vaccine?
Yes.
A highly effective live attenuated vaccine derived from the historic 17D strain is available.
What is the yellow fever vaccine called in the United States?
The licensed vaccine is YF-VAX.
How many yellow fever vaccine doses are needed?
For most eligible people, one dose provides long-lasting protection.
Some special populations may need or be considered for additional doses.
How quickly does the vaccine work?
WHO reports that 80% to 100% of vaccinated people develop protective immunity within approximately 10 days and more than 99% within 30 days.
Is yellow fever vaccination required for international travel?
Some countries require an International Certificate of Vaccination or Prophylaxis.
Requirements depend on the destination, country of origin, travel route, and transit history.
Can pregnant women receive the yellow fever vaccine?
Pregnancy is considered a precaution rather than an automatic recommendation for routine vaccination.
The decision should be based on the actual risk of exposure and the risks and benefits of vaccination.
Can infants receive yellow fever vaccine?
Routine travel vaccination is generally recommended beginning at 9 months of age when indicated.
Age 6 through 8 months is a precaution, and vaccination is contraindicated for younger infants under current CDC recommendations.
Can adults over 60 receive the vaccine?
Yes, in some circumstances.
However, age 60 years or older is a precaution because serious vaccine-associated adverse events occur more frequently in older recipients.
The risks of vaccination and the risks of yellow fever exposure should be considered individually.
Does yellow fever still exist?
Yes.
Yellow fever continues circulating in parts of Africa and South America, and cases and outbreaks were still being reported in 2026.
Does yellow fever occur naturally in the United States today?
Endemic yellow fever transmission does not currently occur in the United States.
Travel-associated cases are possible when unvaccinated people become infected in risk areas abroad.
Can someone get yellow fever twice?
People who recover from infection generally develop long-lasting protective immunity, making repeat clinical infection unlikely.
Why Is Yellow Fever Historically Important?
Few infectious diseases have influenced the development of modern public health as strongly as yellow fever.
For centuries, epidemics killed large numbers of people while physicians struggled to understand how the disease spread.
The history moved through several crucial discoveries.
Yellow fever devastated cities throughout the Americas during the eighteenth and nineteenth centuries.
Carlos Finlay proposed mosquito transmission in 1881.
Walter Reed’s commission and other researchers confirmed mosquito transmission around the beginning of the twentieth century.
Mosquito-control programs then demonstrated that understanding the life cycle of a disease could transform public health.
The yellow fever virus itself was isolated in 1927.
Max Theiler and his colleagues subsequently developed the 17D vaccine in 1937.
The vaccine became one of the most successful viral vaccines ever developed, and Theiler’s work earned the 1951 Nobel Prize in Physiology or Medicine.
Yet yellow fever has not disappeared.
The reason is ecological.
The virus is maintained in mosquito and non-human primate populations, particularly within forest ecosystems. Human vaccination can dramatically reduce illness and prevent urban epidemics, but it cannot remove the virus from these natural cycles.
That makes yellow fever a permanent example of the connection among human health, animal populations, mosquito ecology, international travel, vaccination, and environmental conditions.
Modern medicine has no specific antiviral cure for yellow fever, but it possesses something exceptionally powerful: an effective vaccine capable of preventing the disease before exposure.
For people living in or traveling to genuine risk areas, appropriate vaccination combined with mosquito-bite prevention remains the foundation of protection.
Medical and vaccine information reviewed against CDC, WHO, FDA, and current public health guidance available through September 2026.