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Clinical Features Prevention and Control of Yellow Fever
Vaccination for Yellow Fever
Thermometer Thermometer
Rapid immunization of the population at risk is the most effective control strategy for yellow fever. For international use, the approved vaccine is the 17D vaccine. It is a live, attenuated vaccine prepared from a non–virulent strain (17D strain) which is grown in chick embryos and subsequently freeze–dried. The sensitivity of the lyophilized 17D vaccine to heat is a major drawback in the use of this vaccine in mass campaigns in tropical countries. It has to be stored between +5 and –30 degrees Celcius, preferably below 0 degree celcius, until reconstituted with the sterile, cold, physiological saline dilutant, provided the reconstituted vaccine is kept on ice, away from sunlight, and discarded if not used within half an hour.

The vaccine is administrated subcutaneously at the insertion of deltoid in a single dose of 0.5 ml irrespective of age. Immunity begins to appear on the 7th day and lasts for more than 35 years, and possibly for life. However, WHO recommends re–vaccination after 10 years for international travel. Although no teratogenic effects have been ascribed to the yellow fever vaccine, immunization should be avoided during pregnancy if there is no risk of exposure. Similarly, immunization may be postponed until the child is one year old, if there is no risk of exposure to the yellow fever virus. Mild post–vaccinal reactions (e.g. myalgia, headache and low–grade fever), may occur in 2 to 5% of the vaccines, about five to 10 days after vaccination. Anaphylaxis is very rare, occurring mainly in those allergic to eggs. Cholera and yellow fever vaccines together or within three weeks, interfere with each other. So, whenever possible, they should be administered about three weeks or more apart.

Vector Control
The other principal method of preventing yellow fever is through intensive vector control. The objective of vector control is to reduce the vector population rapidly to the lowest possible level and thereby stop or reduce transmission quickly. This approach has proved successful in the Americas to prevent urban epidemics.

The vector, the Aedes mosquito is peridomestic in habits. It can be controlled by vigorous anti–adult and anti–larval measures. The long term policy should be based on organized “Source reduction” methods (e.g. elimination of breeding places) supported by health education aimed at securing community participation. Personal protection against contact with insects is of major importance in integrated vector control. Such protection may include the use of repellents, mosquito nets, mosquito coils and fumigations mats.

Surveillance of Yellow Fever
A program of surveillance (clinical, serological, histopathological and entomological) should be instituted in countries where the disease is endemic for the early detection of the presence of the virus in human populations or in animals that may contribute to its dissemination.

For the surveillance of the Aedes mosquitoes, the WHO uses an index known as the Aedes aegypti index. This is a house index and defined as “The percentage of houses and their premises, in limited, well defined areas, showing actual breeding of the Aedes aegypti larvae”. This index should not be more than 1% in towns and seaports in endemic areas to ensure freedom from yellow fever.

International measures
India is a yellow–fever “Receptive” area, that is, “An area in which yellow fever does not exist, but where conditions would permit its development if introduced.” The population of India is unvaccinated and hence particularly susceptible to yellow fever. The missing link in the chain of transmission is the virus of yellow fever which does not seem to occur in India.

The virus of yellow fever could get imported into India in two ways Measures designed to restrict the spread of yellow fever have been specified in the ‘International Health Regulations’ of the World Health Organization. These are implemented by the Government of India through stringent aerial and maritime traffic regulations. Broadly, these comprise: International Certificate of Vaccination
India and most other countries require a valid certificate of vaccination against yellow fever from travelers coming from infected areas. A few countries (including India) require this even if the traveler has been in transit. It is up to each country to decide whether a certificate of vaccination against yellow fever shall be required for infants under one year of age, after weighing the risks to the infant arising from vaccination. In this regard, India requires vaccination for infants too. The validity of the certificate begins two days after the date of vaccination and extends up to 10 years.

Re–vaccination performed before the end of validity of the certificate renders the certificate valid for a further period of 10 years starting on the day of re–vaccination. For the purpose of international travel, the vaccination must be given at an officially designated center and the certificate must be validated with the official stamp of the Ministry of Health, Government of India. The certificate is valid only if it conforms to the model prescribed under international health regulations. On the other hand, for their own protection, travelers who enter endemic areas should get themselves vaccinated against yellow fever.

Reference centers
The yellow fever reference centers in India are: The international control of yellow fever remains one of the major medical challenges of this century.