Children
Rates of influenza-associated hospitalization are substantially higher among infants and young children than among older children when influenza viruses are in circulation (Figure 2) and are similar to rates for other groups considered at high risk for influenza-related complications (61--66), including persons aged >65 years (59,63).
Annual hospitalization rates for laboratory-confirmed influenza decrease with increasing age, ranging from 240--720 per 100,000 children aged <6 months to approximately 20 per 100,000 children aged 2--5 years (38). Hospitalization rates for children aged <5 years with high-risk medical conditions are approximately 250--500 per 100,000 children (56,58,67).
An estimated annual average of 92 influenza-related deaths (0.4 deaths per 100,000 persons) occurred among children aged <5 years during the 1990s
Among the 149 children who died and for whom information on underlying health status was available, 100 (67%) did not have an underlying medical condition that was an indication for vaccination at that time (68). In California during the 2003--04 and 2004--05 influenza seasons, 51% of children with laboratory-confirmed influenza who died and 40% of those who required admission to an intensive care unit had no underlying medical conditions (69). These data indicate that although deaths are more common among children with risk factors for influenza complications, the majority of pediatric deaths occur among children of all age groups with no known high-risk conditions.
Deaths among children that have been attributed to co-infection with influenza and Staphylococcus aureus, particularly methicillin resistant S. aureus (MRSA), have increased during the preceding four influenza seasons (70; CDC, unpublished data, 2008). The reason for this increase is not established but might reflect an increasing prevalence within the general population of colonization with MRSA strains, some of which carry certain virulence factors (71,72).
Adults
One retrospective analysis based on data from managed-care organizations collected during 1996--2000 estimated that the risk [of death] during influenza season among persons aged >65 years ...[was]... approximately 190 per 100,000 healthy elderly persons.
Influenza-associated excess deaths among pregnant women were reported during the pandemics of 1918--1919 and 1957--1958 (80--83). Case reports and several epidemiologic studies also indicate that pregnancy increases the risk for influenza complications (84--89) for the mother.
The rate of third-trimester hospital admissions during the influenza season was five times higher than the rate during the influenza season in the year before pregnancy and more than twice as high as the rate during the noninfluenza season.