Public health strategies
Yellow fever is endemic in 33 countries in Africa and 11
countries in South America. There are two modes of transmission of the yellow fever virus,
the sylvatic or forest cycle and the urban cycle. Transmission begins when vector
mosquitos (Aedes africanus in Africa, and several species of the genus Haemagogus
in South America) feed on non-human primates infected with the virus. The infected mosquitos
then feed on humans travelling through the forest. The greatest risk of an epidemic
occurs when viraemic humans return to urban areas and are fed on by the domestic vector
mosquito Aedes aegypti, which then transmits the virus to other humans.
A severe epidemic is most likely to occur if conditions
allow the density of vector populations to increase substantially, as can happen in a
rainy season. Good epidemiological surveillance can be critical in preventing an
Yellow fever continues to be a public health concern in many countries of Africa and the Americas. It is estimated that
200 000 cases and 30 000 deaths are attributable to yellow fever annually, most of them
occurring in sub-Saharan Africa, although far fewer cases than this are reported.
The main strategies to control yellow fever are based
on a combination of immunization for protection against the disease and surveillance, and
are outlined below:
Preventing outbreaks in high-risk areas through mass
Control of Aedes aegypti in urban centres.
- Administering yellow fever vaccine as part of routine
*Both these strategies should ensure a minimum
coverage of at least 80%.
- Instituting a sensitive and reliable YF surveillance
system including laboratory capacity to analyse samples and confirm suspected cases.
- Emergency response to outbreaks through mass campaigns.
For the 33 countries of equatorial Africa where yellow
fever is endemic, which have a combined population of 508 million, the vaccine should be
routinely administered at the same time as measles vaccine, i.e. around nine months of
age. Immunization services and disease-reporting systems are well established in these
countries, all of which are committed to the goals of measles reduction and polio
eradication. Improvement in disease surveillance is expected to follow and to be
sustainable. Linking yellow fever to planned polio and measles immunization activities
could save thousands of lives each year.
The incorporation of yellow fever vaccine into the
routine infant and child immunization schedules was recommended in 1988 by a joint
WHO/UNICEF Technical Group on Immunization in Africa. It was suggested that this be done
at the time of the visit for measles vaccine (at 9 to 12 months of age), thus avoiding the
need for an additional visit. As at the end of 2001, 15 of the 33 at-risk countries had
implemented the recommendation. At the end of 2000, 10 countries in Africa reported
coverage by the age of 12 months, the estimated average being 42%. In the at-risk
countries of the Americas, 204 cases and 97 deaths were reported in 1999. After this, a
more aggressive implementation contributed to a significant reduction in the number of
cases from the Americas (102 cases and 51 deaths in 2000, and 80 cases and 46 deaths in
International health regulations: A yellow fever
vaccination certificate is now the only vaccination certificate that should be required in
international travel, and then only for a limited number of persons. Many countries
require a valid international certificate of vaccination from travellers, including those
in transit, arriving from infected areas or from countries with infected areas. Some
countries require a certificate from all entering travellers, even those arriving from
countries where there is no risk of yellow fever. Although this exceeds the provisions of
International Health Regulations, travellers may find that it is strictly enforced,
particularly for people arriving in Asia from Africa or South America. Vaccination is strongly advised for travellers outside
urban areas of countries in zones where yellow fever is endemic, even if these countries
have not officially reported the disease and do not require evidence of vaccination on
entry. The actual areas of yellow fever virus activity far exceed the officially reported
Vaccine supply: Efforts are
being made to ensure an adequate supply of vaccine so as to permit routine immunization,
preventive campaigns and outbreak response in countries of endemicity. Until recently the
vaccine was in short supply. A global stockpile currently exists for use in emergencies.
Contraindications: The vaccine is
contraindicated in immune-deficient patients, in individuals allergic to eggs and before
six months of age. Individuals with symptomatic HIV infection should not receive yellow
fever vaccine until such time as more information is available on its safety. Some travel
clinics decide whether to administer the vaccine on the basis of the CD4 count. The risk
of exposure to disease must be weighed against the potential risk of the vaccine during
Adverse events: Very rare cases of serious
adverse events, including deaths, have recently been reported. The risk to unimmunized
individuals either living in or travelling to areas where there is known yellow fever
transmission is far greater than the risk of having a vaccine-related adverse event. Therefore, WHO policy on yellow fever vaccination remains
Number of doses
One dose of 0.5 ml subcutaneously
Routine immunization with measles vaccine at nine months
International health regulations require a booster every
Egg allergy; immune deficiency from medication or
disease; symptomatic HIV infection; hypersensitivity to previous dose; pregnancy*
Hypersensitivity to egg; rarely, encephalitis in the very
young; hepatic failure. Rare reports of death from massive organ failure (see above).
Do not give before six months of age; avoid during
* To be weighed according to risk of exposure and term of
Adverse events following yellow fever vaccination. Weekly
Epidemiological Record, 2001, 76(29):217218, and on the Internet at www.who.int/wer/pdf/2001/wer7629.pdf.
Monath TP. Yellow fever: an update. Lancet,
Infectious Diseases 1:11-20, 2001.
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 and on the Internet at
International travel and health. Geneva: World
Health Organization; 2002 and on the Internet at www.who.int/ith.
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.
The immunological basis for immunization. Module 8:
Yellow fever (S. Robertson). Geneva, 1993 (unpublished documents WHO/EPI/GEN/93.18;
available from Vaccines and Biologicals, World Health Organization, 1211 Geneva 27,
Switzerland and on the Internet at www.who.int/vaccines-documents/DocsPDF-ibi-e/mod8_e.pdf).
Yellow Fever. Technical Consensus Meeting Geneva, 2-3
March 1998. Geneva, 1998 (unpublished document WHO/EPI/GEN/98.08; available from
Vaccines and Biologicals, World Health Organization, 1211 Geneva 27, Switzerland and on
the Internet at www.who.int/vaccines-documents/DocsPDF/www9833.pdf).
Yellow fever. Geneva, 1998 (unpublished document
WHO/EPI/GEN/98.11; available from Vaccines and Biologicals, World Health Organization,
1211 Geneva 27, Switzerland and on the Internet at www.who.int/vaccines-documents/DocsPDF/www9842.pdf).
Fact Sheet on yellow fever
guidelines for yellow fever surveillance
CDC home page for Yellow
Updated May 2003
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