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Pathogenesis, Clinical Features and Diagnosis

Figure 20 Influenza and pneumonia mortality by age in the USA. Influenza- and pneumonia-specific mortality by age, including the pandemic year 1918 and the average of the interpandemic years 1911–15, is shown. Specific death rate is per 100,000 of the population for each age division. Purple line, 1918 pandemic; orange line, average of interpandemic years 1911–15. source: Adapted from Potter CW, editor. Influenza (Perspectives in Medical Virology), 2003 with permission from Elsevier.


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  • As shown in Figure 20, death was age-related during interpandemic years preceding the Spanish flu outbreak, with high mortality rates in the under 5-year-olds and the elderly.
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Figure 21 Influenza symptoms and their associated frequencies. source: Data are from 10 studies involving 520 adults with uncomplicated influenza A. Adapted from Nicholson KG. Human influenza. In: Nicholson KG, Webster RG, Hay AJ, editors. Textbook of Influenza. Oxford: Blackwell Science, 1998; pp. 219–264 with permission from Blackwell Publishing.


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  • The frequency of influenza symptoms in adults, shown in Figure 21, may vary with the age of the patient.
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Table 6 People at high risk of hospitalization or death from influenza. source: Adapted from World Health Organization. Influenza vaccines. Wkly Epidemiol Rec 2000;75:281–288.

People at high risk of hospitalization or death from influenza
People in specific age categories
  • adults aged 65 years and older
  • children under age 2 years
People with chronic disease
  • chronic pulmonary disease (asthma, bronchitis, emphysema, tuberculosis)
  • chronic cardiovascular disease
  • diabetes mellitus
  • renal disease
Immunocompromised people
  • transplant recipients
  • HIV-positive people
  • splenic dysfunction
Pregnant women

References in context

  • Table 6 summarizes the high-risk populations for significant morbidity and mortality from influenza.
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Table 7 Criteria for clinical diagnosis of influenza. source: Reproduced from Nicholson KG. Managing Influenza in Primary Care, 1999 with permission from Blackwell Publishing.

Criteria for clinical diagnosis of influenza
Fever (≥ 37.8°C) and/or feverishness
TWO of:
  • Cough
  • Nasal symptoms
  • Headache
  • Sore throat
  • Myalgia
  • Malaise

References in context

  • Table 7 lists the criteria for purposes of clinical diagnosis of influenza that have been used in trials of antiviral drugs.
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