Symptoms and Treatment
Symptoms

• The majority of persons infected with yellow fever virus have no illness or only mild illness.
• In persons who develop symptoms, the incubation period (time from infection until illness) is typically 3–6 days.
• The initial symptoms include sudden onset of fever, chills, severe headache, back pain, general body aches, nausea, and vomiting, fatigue, and weakness. Most persons improve after the initial presentation.
• After a brief remission of hours to a day, roughly 15% of cases progress to develop a more severe form of the disease. The severe form is characterized by high fever, jaundice, bleeding, and eventually shock and failure of multiple organs.

Treatment

• No specific treatments have been found to benefit patients with yellow fever. Whenever possible, yellow fever patients should be hospitalized for supportive care and close observation.
• Treatment is symptomatic. Rest, fluids, and use of pain relievers and medication to reduce fever may relieve symptoms of aching and fever.
• Care should be taken to avoid certain medications, such as aspirin or other nonsteroidal anti-inflammatory drugs (e.g. ibuprofen, naproxen), which may increase the risk of bleeding.
• Yellow fever patients should be protected from further mosquito exposure (staying indoors and/or under a mosquito net) during the first few days of illness. This way, yellow fever virus in their bloodstream will be unavailable to other uninfected mosquitoes, thus breaking the transmission cycle and reducing risk to the persons around them.

Outcome

• The majority of infected persons will be asymptomatic or have mild disease with complete recovery.
• In persons who become symptomatic but recover, weakness and fatigue may last several months.
• Among those who develop severe disease, 20% - 50% may die. 
• Those who recover from yellow fever generally have lasting immunity against subsequent infection.

Prevention of Yellow Fever 

Avoid Mosquito Bites

• Use insect repellent. When you go outdoors, use an EPA-registered insect repellent such as those containing DEET, picaridin, IR3535, or oil of lemon eucalyptus on exposed skin. Even a short time outdoors can be long enough to get a mosquito bite. For details on when and how to apply repellent, see Insect Repellent Use and Safety. 
• Wear proper clothing to reduce mosquito bites. When weather permits, wear long-sleeves, long pants and socks when outdoors. Mosquitoes may bite through thin clothing, so spraying clothes with repellent containing permethrin or another EPA-registered repellent will give extra protection. Clothing pre-treated with permethrin is commercially available. Mosquito repellents containing permethrin are not approved for application directly to skin.
• Be aware of peak mosquito hours. The peak biting times for many mosquito species is dusk to dawn. However, Aedes aegypti, one of the mosquitoes that transmits yellow fever virus, feeds during the daytime. Take extra care to use repellent and protective clothing during daytime as well as during the evening and early morning. Staying in accommodations with screened or air-conditioned rooms, particularly during peak biting times, will also reduce risk of mosquito bites.

Transmission of Yellow Fever Virus
Yellow fever virus is an RNA virus that belongs to the genus Flavivirus. It is related to West Nile, St. Louis encephalitis, and Japanese encephalitis viruses. Yellow fever virus is transmitted to humans primarily through the bite of infected Aedes or Haemagogus species mosquitoes. Mosquitoes acquire the virus by feeding on infected primates (human or non-human) and then can transmit the virus to other primates (human or non-human). Humans infected with yellow fever virus are infectious to mosquitoes shortly before the onset of fever and for 3–5 days after onset.
Yellow fever virus has three transmission cycles: jungle (sylvatic), inter­mediate (savannah), and urban.

• The jungle (sylvatic) cycle involves transmission of the virus between nonhuman primates (e.g., monkeys) and mosquito species found in the forest canopy. The virus is transmitted by mosquitoes from monkeys to humans when humans are visiting or working in the jungle.
• In Africa, an intermediate (savannah) cycle exists that involves transmission of virus from mosquitoes to humans living or working in jungle border areas. In this cycle, the virus can be transmitted from monkey to human or from human to human via mosquitoes.
• The urban cycle involves trans­mission of the virus between humans and urban mosquitoes, primarily Aedes aegypti. The virus is usually brought to the urban setting by a viremic human who was infected in the jungle or savannah.

Yellow Fever Vaccine

Yellow fever vaccine is a live-virus vaccine which has been used for several decades. A single dose protects against disease for 10 years or more. If a person is at continued risk of infection, a booster dose is recommended every 10 years.

Yellow Fever Vaccine Recommendations

Yellow fever vaccine is recommended for persons aged ≥9 months who are traveling to or living in areas at risk for yellow fever virus transmission in South America and Africa. Yellow fever vaccine may be required for entry into certain countries. Yellow fever vaccination requirements and recommendations for specific countries are available on the CDC Travelers' Health page.
Serious adverse events can occur following yellow fever vaccination. Therefore, persons should only be vaccinated if they are at risk of exposure to yellow fever virus or require proof of vaccination for country entry. To minimize the risk of serious adverse events, health-care providers should carefully observe the contraindications and consider the precautions about vaccination prior to vaccine administration. Individuals considering yellow fever vaccination should discuss their underlying health conditions with their health care provider. A medical waiver can be given for persons with a precaution about or contraindication to vaccination. More information about medical waivers is available on the CDC Travelers' Health website.
To minimize the risk of serious adverse events, health-care providers should carefully observe the contraindications and consider the precautions about vaccination prior to vaccine administration (see below).
For more information about the use of yellow fever vaccine in travelers or laboratory workers, see the Advisory Committee on Immunization Practice (ACIP) recommendations.
This vaccine is administered only at designated vaccination centers. Locations of centers are available from your local health department. Information regarding registered yellow fever vaccination sites can be viewed at the CDC Travelers' Health Yellow Fever website. For health-care professionals who want to learn more about yellow fever disease and vaccine, a continuing education module is available.

Contraindications (conditions in which vaccine should not be given)

• Allergy to vaccine component
• Age <6 months
• Symptomatic HIV infection or CD4+ T-lymphocytes <200/mm3 (<15% of total in children aged <6 years)
• Thymus disorder associated with abnormal immune function
• Primary immunodeficiencies
• Malignant neoplasms
• Transplantation
• Immunosuppressive and immunomodulatory therapies

Precautions (conditions for which the risks of the vaccine and the disease should be carefully considered)

• Age 6 to 8 months
• Age ≥60 years
• Asymptomatic HIV infection and CD4+ T-lymphocytes 200 to 499/mm3 (15-24% of total in children aged <6 years)
• Pregnancy
• Breastfeeding

Reactions to Yellow Fever Vaccine

Reactions to yellow fever vaccine are generally mild and include headaches, myalgia (muscle aches), and low-grade fevers. There have been reports of rare but serious events following yellow fever vaccination. These events include anaphylaxis (life-threatening allergic reaction), yellow fever vaccine-associated viscerotropic disease (YEL-AVD, disease affecting certain internal organs), and yellow fever vaccine-associated neurologic disease (YEL-AND, disease affecting the nervous system). CDC can provide laboratory testing support for suspected YEL-AVD and YEL-AND cases.