Prior to the availability of varicella vaccine there were approximately 4 million cases of varicella a year in the U.S. Though usually a mild disease in healthy children, an estimated 150,000 to 200,000 people developed complications, about 11,000 people required hospitalization and 100 people died each year from varicella. Varicella tends to be more severe in infants, adolescents and adults than in young children. The most common complications from varicella include bacterial superinfection of skin lesions, pneumonia, central nervous system involvement, and thrombocytopenia.