Vaccine Information

Entry last updated: May 28, 2004

- Understanding the Disease
- Available Vaccines
- History of the Vaccine
- Who Should and Should Not Receive the Vaccine
- Dose Schedule
- Effectiveness of the Vaccine
- Known Side Effects
- Related Issues
- Key References and Sources of Additional Information

Understanding the Disease

Yellow fever is caused by a virus that is carried by a biting mosquito. Only humans and monkeys can be naturally infected with yellow fever virus. A mosquito that is capable of transmitting yellow fever virus is present in southern parts of the United States.

Epidemic yellow fever used to occur in the United States but now the disease occurs only in sub-Saharan Africa and tropical South America, where it is endemic and intermittently epidemic. There are 200,000 estimated cases of yellow fever (with 30,000 deaths) per year. However, due to underreporting, only a small percentage of these cases are identified. Small numbers of imported cases also occur in countries free of yellow fever.

Infection causes a wide spectrum of disease. Most cases of yellow fever are mild and similar to influenza, consisting of fever, headache, nausea, muscle pain and prominent backache. After three to four days most patients improve and their symptoms disappear. However, in about 15% of patients, fever reappears after 24 hours with severe illness which includes hepatitis and hemorrhagic fever. Bleeding can occur from the mouth, nose, eyes and/or stomach. Once this happens, blood appears in the vomit and feces. Kidney function also deteriorates. Half of those who develop the severe illness die within 10-14 days. The remainder recovers without significant organ damage.

The "yellow" in the name is explained by the jaundice that occurs with hepatitis.

Available Vaccines

Product: YF-VAX� (Live-attenuated 17D strain vaccine)
Manufacturer: Aventis Pasteur

There is no thimerosal in yellow fever vaccine.

History of the Vaccine

Yellow fever vaccine is an attenuated, live virus vaccine that has been used since the 1930s.

The isolation of the Asibi and French strains of yellow fever in 1927 enabled the development of vaccines. Scientists at the Rockefeller Foundation in New York developed a live vaccine (17D) attenuated by serial passage of the Asibi strain in embryonated chicken eggs. The 17D vaccine was first tested in 1936 in New York and in 1937 in Brazil.

Although the vaccine has been available for more than 60 years, the number of people infected over the last two decades has increased and yellow fever is now a serious public health issue in a number of countries once again.

Who Should and Should Not Receive this Vaccine

Who should receive the yellow fever vaccine?

• All persons 9 months or older of age traveling to or living in areas where yellow fever infection is reported or yellow fever vaccination is required (specific risks for exposure to yellow fever in various regions of the world are available at www.cdc.gov/travel/destinations )

The vaccine is recommended for all travelers passing through or living in countries in Africa, Central America and South America where yellow fever infection is officially reported. It also is recommended for travel outside the urban areas of countries that do not officially report yellow fever but lie in the yellow fever �endemic zones.�

A vaccination certificate is required for entry to many countries, particularly for travelers arriving in Asia from Africa or South America. Fatal cases in unvaccinated tourists have been reported with an average of one a year for the past ten years.

Who should not receive the yellow fever vaccine?

• Yellow fever vaccine should not be given to infants under 6 months of age due to a risk of viral encephalitis (brain inflammation) developing in the child. In most cases, vaccination should be deferred until the child is 9 to 12 months of age to minimize the risk of vaccine associated encephalitis. ��
• Pregnant women should not be vaccinated because of a theoretical risk that the developing fetus may become infected from the vaccine. �
• Infants 6 to 9 months of age and pregnant women should be considered for immunization only if they are traveling to high-risk areas, travel cannot be postponed and a high level of prevention against mosquito exposure is unfeasible.
• Persons hypersensitive to eggs should not receive the vaccine because it is prepared in embryonated eggs.�
• Immunocompromised persons and persons receiving cancer chemotherapy or high doses of steroids. People with asymptomatic HIV infection may be vaccinated if exposure to yellow fever cannot be avoided.

Dose Schedule

The yellow fever vaccine is administered to persons of all ages as a single dose. Immunity develops by the 10th day after primary vaccination.

A booster vaccination should be administered after 10 years. Revaccination boosts antibody titer; however, evidence from several studies suggests that yellow fever vaccine immunity persists for at least 30 to 35 years and probably for life.

Studies have shown that the serologic response to yellow fever vaccine is not inhibited by administration of certain other vaccines concurrently at separate sites or at various intervals of a few days to one month. Measles and yellow fever vaccines have been administered in combination with full efficacy of each of the components; Bacillus Calmette Gu�rin (BCG) and yellow fever vaccines have been administered simultaneously without interference; typhoid, meningococcal and yellow fever vaccines have been administered concurrently with full efficacy of each of the components.

Effectiveness of the Vaccine

Vaccination is the single most important measure for preventing yellow fever. In populations where vaccination coverage is low, vigilant surveillance is critical for prompt recognition and rapid control of outbreaks. Mosquito control measures can be used to prevent virus transmission until vaccination has taken effect.

Yellow fever vaccine is safe and highly effective. The protective effect (immunity) occurs within one week in 95% of people vaccinated. A single dose of vaccine provides protection for 10 years and probably for life.

Known Side Effects

Yellow fever vaccine generally has few side effects; fewer than 5% of vaccinees develop mild headache, muscle pain, or other minor symptoms 5 to 10 days after vaccination.

Reactions to yellow fever vaccine are generally mild. After vaccination, vaccinees have reported mild headaches, myalgia, low-grade fevers, or other minor symptoms 5�10 days after vaccination. In clinical trials, the incidence of mild adverse events has been ~25%.

Approximately 1% of vaccinees find it necessary to curtail regular activities. Immediate hypersensitivity reactions, characterized by rash, urticaria, or asthma or a combination of these, are uncommon (incidence <1 case per 131,000 vaccinees) and occur principally in persons with histories of egg allergy.

Rarely yellow fever vaccine can cause multiple organ system failure and encephalitis.

Related Issues

Over 400 million doses of yellow fever vaccine have been given and serious side effects are rare. However, recently a few serious adverse outcomes, including deaths, have been reported in Brazil, Australia and the United States. Scientists are investigating the cause of these adverse events, and monitoring to ensure detection of any similar incidents.

The risk to life from yellow fever is far greater than the risk from the vaccine, so people who may be exposed to yellow fever should be protected by immunization. If there is no risk of exposure, for example, if a person will not be visiting an endemic area, there is no necessity to receive the vaccine. Since most of the other known side effects have occurred in children less than six months of age, vaccine is not administered to this age group. The vaccine should only be given to pregnant women during vaccination campaigns in the midst of an epidemic.

Key References and Sources of Additional Information

•  Aventis Pasteur. YF-VAX: Yellow Fever Vaccine (Product Information, May 2001).
• CDC. Adverse events associated with 17D-derived yellow fever vaccination � United States, 2001-2002. MMWR 2002; 51: 989-93.
• District guidelines for yellow fever surveillance. Geneva, 1998 (unpublished document WHO/EPI/GEN/98.09; available from Vaccines and Biologicals, World Health Organization, 1211 Geneva 27, Switzerland.
• Monath TP. Yellow fever: an update. Lancet, Infectious Diseases 1:11-20, 2001.  
• National Center for Infectious Diseases, CDC. The Yellow Book: Health Information for International Travel, 2003-2004.
• Silva J, Cerqueira R, Sousa Ma L, Luna E. Vaccine safety: yellow fever vaccine. Report of the Technical Advisory Group on Vaccine Preventable Disease. Washington DC: Pan American Health Organization; 2000. 
• Plotkin SA, Orenstein WA (Eds). Vaccines (4th Edition). Philadelphia, PA: W.B. Saunders Company, 2004 
• World Health Organization. Adverse events following yellow fever vaccination. Weekly Epidemiological Record, 2001, 76(29):217�218. 
• World Health Organization (2004). Health Topics: Yellow Fever.

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