Study to Evaluate the Immune Response to Pneumococcal Vaccine in Alefacept-treated Adults With Chronic Plaque Psoriasis

Inclusion Criteria: * Subject has chronic plaque psoriasis * Subject has at least 5% body surface area affected with psoriasis Exclusion Criteria: * Skin disorder other than plaque psoriasis in affected area * Previously immunized with any pneumococcal vaccine * Subject has pustular or erythrodermic psoriasis * Subject is immunocompromised * Six or more herpes simplex virus outbreaks per year * History or malignancy, chronic serious infection or hepatic disease