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rily with increased mortality in the elderly population. Therefore, influenza encompasses a variety of clinical responses ranging from asymptomatic or mild respiratory infection to primary viral pneumonia or secondary bacterial pneumonia with fatal outcome.

Recently, epidemics have alternated between those caused primarily by type A and those caused by type B. Both are transmitted by sneezing, coughing, speaking, and also by direct contact through small-particle aerosols. Transmission usually occurs during the initial stages when infected individuals shed substantial amounts of the virus through respiratory secretions. The episodes of winter influenza are partly explained by the ability of small droplets to remain infectious in the cold and in low humidity.

DISEASE BURDEN

Epidemiology

For the purposes of the calculations in this report, the committee estimated that there are approximately 54,000,000 cases of influenza A and B each year in the United States. Incidence rates in children under 14 years of age are over twice that in adults 35 years of age and older. There were approximately 42,250 deaths each year due to influenza, with very high mortality in people 65 years of age and older. See Table A12–1.

Disease Scenarios

For the purposes of the calculation in this report, the committee assumed that 98% of influenza infections are associated with a moderate to severe respiratory illness not requiring hospitalization. It was assumed that most of these infections require only 3 days of bed rest and 2 weeks of mild recovery. Approximately 10% of infections are associated with a more serious sinusitis in conjunction with the 2-week recovery. It was assumed that approximately 5% of influenza infections are associated with a 3-month period of fatigue in addition to the scenario described above. It was assumed that 2% of influenza infections result in hospitalization for pneumonia. It was further assumed that a small number (.1%) of influenza infections exacerbate underlying cardiac or pulmonary conditions. This exacerbation of chronic disease was assumed to be associated with an extra disease burden of 8.5 days of an HUI of .53. See Table A12–2.

COST INCURRED BY DISEASE

Table A12–3 summarizes the health care costs incurred by influenza A and B infections. For the purposes of the calculations in this report, it was assumed



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