Yellow fever occurs only in Africa and South America. The clinical spectrum of yellow fever ranges from subclinical infection to overwhelming pansystemic disease. Yellow fever has an abrupt onset after an incubation period of 3–6 days, and usually includes fever, prostration, headache, photophobia, lumbosacral pain, extremity pain (including the knee joints), epigastric pain, anorexia, and vomiting. The illness might progress to liver and renal failure, and hemorrhagic symptoms and signs caused by thrombocytopenia and abnormal clotting and coagulation can occur. The fatality rate of severe yellow fever is approximately 20%.
Treatment for yellow fever consists of providing general supportive care and varies, depending on which organ systems are involved. No effective specific antiviral therapy for yellow fever has been identified.
Two forms of yellow fever, urban and jungle, are epidemiologically distinguishable. Clinically and etiologically they are identical. Urban yellow fever is an epidemic viral disease of humans transmitted from infected to susceptible persons by Aedes aegypti mosquitoes, which breed in domestic and peridomestic containers (e.g., water jars, barrels, drums, tires, or tin cans) and thus in close association with humans. In areas where Ae. aegypti has been eliminated or suppressed, urban yellow fever has disappeared.
Urban yellow fever can be prevented by vaccinating human populations at risk for infection or by suppressing populations of Ae. aegypti mosquitoes so that they no longer perpetuate infection. Jungle yellow fever can most effectively be prevented by vaccination of human populations at risk for exposure.
Yellow fever vaccine is a live, attenuated virus preparation made from the 17D yellow fever virus strain. Historically, the 17D vaccine has been considered to be one of the safest and most effective live virus vaccines ever developed.
The 17D yellow fever vaccine virus family is the foundation for both the 17D-204 lineage and 17DD lineage. Vaccine type 17D-204 is used in both the United States and Australia, whereas vaccine type 17DD is used in Brazil. The two vaccine types share 99.9% sequence homology.
The vaccine should be stored at temperatures of 2ºC–8ºC (35ºF–46ºF) until it is reconstituted by the addition of diluent (sterile, physiologic saline) supplied by the manufacturer. Multidose vials of reconstituted vaccine should be held at 2ºC–8ºC (35ºF–46ºF); unused vaccine should be discarded within 1 hour after reconstitution.
DOSAGE AND ADMINISTRATION
Persons aged >9 months who are traveling to or living in areas of South America and Africa where yellow fever infection is officially reported should be vaccinated. Vaccination is also recommended for travel to countries that do not officially report the disease but that lie in the yellow fever-endemic zone
Vaccination of infants aged <9 months be avoided because of the risk for encephalitis, and that travel of such persons to countries in yellow fever-endemic zones or to countries experiencing an epidemic be postponed or avoided, whenever possible. Using yellow fever vaccine among infants aged <9 months has not been formally evaluated.
Despite the lack of such information, ACIP and WHO recognize that situations occur in which vaccination of an infant aged <9 months might be considered. One such situation is the unavoidable exposure of children aged 6–8 months to an environment where an increased likelihood of becoming infected with the yellow fever virus exists (e.g., a setting of endemic or epidemic yellow fever). Because of the risk for encephalitis, in no instance should infants aged <6 months receive yellow fever vaccine.
Because the seroconversion rate after vaccination of pregnant women might be markedly reduced compared with that of other healthy women, serologic tests to determine if a specific yellow fever immune response exists should be considered.
Laboratory personnel who might be exposed to virulent yellow fever virus or to concentrated preparations of the 17D vaccine strain by direct or indirect contact or by aerosols should also be vaccinated.
For purposes of international travel, yellow fever vaccines produced by different manufacturers worldwide must be approved by WHO and administered at an approved yellow fever vaccination center. Vaccinees should receive an International Certificate of Vaccination that has been completed, signed, and validated with the center’s stamp when the vaccine is administered.
Certain countries in Africa require evidence of vaccination from all entering travelers. Other countries might waive the requirements for travelers coming from areas where no evidence exists of substantial risk for yellow fever and who are staying <2 weeks.
Travelers should consult CDC’s travel information website at http://www.cdc.gov/travel to determine requirements and regulations for vaccination.
For persons of all ages for whom vaccination is indicated, a single subcutaneous injection of 0.5 mL of reconstituted vaccine is used.
The International Health Regulations require revaccination at intervals of 10 years. Revaccination can boost antibody titer; however, evidence from multiple studies demonstrates that yellow fever vaccine immunity persists for 30– 35 years and probably for life.
Reactions to 17D yellow fever vaccine are typically mild. After vaccination, vaccinees have reported mild headaches, myalgia, low-grade fevers, or other minor symptoms for 5–10 days. In clinical trials, where symptoms are actively elicited, incidence of mild adverse events has been <25% . Approximately 1% of vaccinees curtail regular activities.
Immediate hypersensitivity reactions, characterized by rash, urticaria, or asthma, are uncommon (i.e., an estimated incidence of 1/130,000–250,000) and occur principally among persons with histories of allergies to egg or other substances.
Gelatin is used as a stabilizer in different vaccines, including yellow fever vaccine. Gelatin has been implicated as a cause of allergic reaction related to other vaccines and, therefore, might also do the same regarding yellow fever vaccine.
Vaccine strain viremia after primary vaccination with yellow fever vaccine frequently occurs among healthy persons, but is usually waning or absent after the first week.
Yellow Fever Vaccine-Associated Neurotropic Disease.
Historically, yellow fever vaccine-associated neurotropic disease (formerly known as postvaccinal encephalitis) among children has been the most common serious adverse event associated with yellow fever vaccines.
The occurrence of vaccine-associated neurotropic disease does not appear to be confined to infants but does appear to be limited. The risk for vaccine-associated neurotropic disease has been estimated as <1/8,000,000 persons.