every 300 people over the age of 65 die of influenza during the epidemic). The excess mortality in the elderly caused by influenza epidemics is illustrated in Fig. 4.16, in which the different strains of influenza A or of influenza B responsible for the epidemics are indicated. Note the large increase in mortality during the epidemic of 1957 caused

FIGURE 4.16 Excess mortality caused by influenza A and B virus in the United States between 1934 and 1990. "1935" refers to the winter of 1934—1935. Excess mortality due to the three dominant subtypes of influenza A and influenza B are indicated by the colors shown in the key. Cross-hatched bars are excess mortality in years when both A and B viruses circulated. In 1955 and 1965, type H2N2 circulated with B, in 1983 and 1988 type H1N1 circulated with B, and in 1985 H3N2 circulated with B. Note that the excess mortality of 70,000 in 1957 meant that 0.3% of the U.S. population that was over 65 years old died of influenza in that year. [Redrawn from Fields et al. (1996, p. 1421).]

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