Hypersensitivity to eggs, & chicken proteins. Serious hypersensitivity reactions (eg, anaphylaxis) after a previous dose of any yellow fever vaccine. Immunosuppression whether congenital, idiopathic or acquired, or as a result of treatment w/ high-dose systemic steroids, RT or cytotoxic drugs. History of thymus dysfunction (including myasthenia gravis, thymoma, thymectomy). Symptomatic or asymptomatic HIV infection w/ evidence of impaired immune function. Moderate or severe febrile or acute illness. Infant <9 mth.