Yellow Fever Vaccine

Yellow fever vaccine is a live attenuated vaccine prepared from the 17D-204 strain of yellow fever virus. The vaccine is cultured in pathogen-free chicken embryos, lyophilized (freeze-dried), and reconstituted with sterile 0.9% sodium chloride solution immediately before administration. After reconstitution, the vaccine appears as a slight pink-brown suspension. As a live vaccine, it stimulates both humoral (antibody) and cellular (T-cell) immune responses, providing robust and durable protection against yellow fever infection.

Yellow fever vaccination is indicated for travelers aged 9 months and older visiting areas where yellow fever transmission occurs or where entry requirements mandate proof of vaccination. Specifically, vaccination is recommended for individuals traveling to: - Sub-Saharan Africa and endemic regions, - South America and Central America, - Areas designated by the World Health Organization with evidence of persistent or periodic yellow fever virus transmission, - Countries requiring evidence of vaccination as a condition of entry. Vaccination may be considered for infants aged 6 to 9 months if travel to high-risk endemic areas is unavoidable and the risk of yellow fever infection is substantial. Healthcare workers and laboratory personnel with potential exposure to yellow fever virus may also require vaccination based on occupational risk assessment.

Absolute contraindications preclude vaccination in the following circumstances: - Age under 6 months: Significantly elevated risk of vaccine-associated encephalitis (YEL-AND), with historical data demonstrating 13 of 15 reported encephalitis cases occurring in infants under four months of age. - Hypersensitivity to vaccine components: Severe allergic reaction or anaphylaxis to eggs, chicken protein, gelatin, or any other vaccine ingredient. - Thymus disorder or history of thymectomy: Includes thymoma, myasthenia gravis, DiGeorge syndrome, and any surgical removal of the thymus gland for any reason. Risk of vaccine-associated viscerotropic disease (YEL-AVD) is approximately 6 times higher in this population. - Symptomatic HIV infection or severe immunosuppression: CD4+ count <200 cells/mm³ or <15% of total lymphocytes in children younger than 6 years. - Active malignant neoplasm: Particularly hematologic malignancies such as leukaemia and lymphoma. - Organ transplantation: Current or recent (<6 months) immunosuppressive therapy following solid organ transplant. - Primary immunodeficiencies: Severe combined immunodeficiency (SCID), Wiskott-Aldrich syndrome, and other cellular immune deficiencies.