Wednesday, April 29, 2009

Flu Pandemic Morbidity / Mortality

Pandemic years are associated with many more cases of influenza and a higher case fatality rate than that seen in seasonal flu outbreaks. It is common to encounter clinical attack rate ranges for seasonal flu of 5% to 15% in the literature. For pandemic flu, clinical attack rates are reported in the range of 25% to 50%.

During a typical year in the United States, 30,000 to 50,000 persons die as a result of influenza viral infection. Frequently cited numbers are 20,000 deaths each year, and 37,000 annual deaths. About 5-10% of hospitalizations for influenza lead to fatal outcome in adults.

In normal years, although most influenza infection is in children, the serious morbidity and mortality is almost entirely among elderly people with underlying chronic disease. During influenza epidemics from 1979-80 through 2000-01, the estimated overall number of influenza-associated hospitalizations in the United States ranged from approximately 54,000 to 430,000/epidemic. An average of approximately 226,000 influenza-related excess hospitalizations occurred per year, with 63% of all hospitalizations occurring among persons aged > 65 years.

Influenza-related deaths can result from pneumonia and from exacerbations of cardiopulmonary conditions and other chronic diseases. Deaths of older adults account for > 90% of deaths attributed to pneumonia and influenza. In one study of influenza epidemics, approximately 19,000 influenza-associated pulmonary and circulatory deaths per influenza season occurred during 1976-1990, compared with approximately 36,000 deaths during 1990--1999. Estimated rates of influenza-associated pulmonary and circulatory deaths/100,000 persons were 0.4--0.6 among persons aged 0-49 years, 7.5 among persons aged 50--64 years, and 98.3 among persons aged > 65 years.


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