ABSTRACT
Influenza epidemics are regularly associated with excess rates of pneumonia-and influenza-associated hospitalizations [1,2]. Both influenza A (H3N2 and H1N1) and B can cause severe illness [3], although the highest mortality rates in recent years have generally been seen during years with significant H3 virus activity [4]. Complications and deaths from influenza are of particular concern in those with certain high-risk medical conditions, including adults and children with cardiovascular and pulmonary conditions, or those requiring regular medical care because of chronic metabolic, renal, blood, or immune diseases [5]. Influenza may cause more severe disease and increased hospitalization rates in individuals with human immunodeficiency virus infection [6,7] or iatrogenic immunosuppression [8], and women in the second or third trimesters of pregnancy [9]. Influenza is increasingly recognized as an important health problem in young children. Rates of influenza-related hospitalizations are particularly high in healthy children under 2 years of age, in whom rates approach those of older children with high-risk conditions [10,11]. In addition, a high rate of secondary complications, particularly otitis media and pneumonia, occur in children with influenza infection [12].
