Essential Evidence

Last Updated on 2022-06-24 © 2022 John Wiley & Sons, Inc.

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Mark H. Ebell, MD, MS, Professor, College of Public Health, University of Georgia

Randall Forsch, MD, MPH, Assistant Professor, University of Michigan
Mark H. Ebell, MD, MS, Professor, College of Public Health, University of Georgia
Kenny Lin, MD, MPH, Professor of Family Medicine, Georgetown University

Overall Bottom Line

  • Yearly influenza vaccination is recommended by ACIP (US) for persons over the age of 6 months. A
  • Patients classically present with rapid onset of fever, cough, chills or rigors, sore throat, and myalgias. A clinical decision rule can be used to identify patients at low, moderate or high risk of influenza. B
  • Rapid antigen tests are most useful during the beginning and end of the flu season. C
  • Treatment is largely supportive; antiviral medications are recommended only if the likelihood of influenza is high and the patient presents within 24 hours of symptom onset. There is no evidence that they reduce serious complications or hospitalization. A
  • Predictors of adverse outcome in the elderly include increased age, previous hospitalization, comorbidities, gender, and number of outpatient visits in the previous year (Table 1). A


Influenza is an acute lower respiratory tract infection with systemic manifestations caused by influenza virus and occurring most frequently in epidemics during the winter months.


  • Onset of the flu season varies but typically begins at about week 40 of the year in the northern hemisphere (early October) and peaks in January. 45

Other Impact

  • Pneumonia and influenza typically cause between 6% and 8% of deaths in the United States but may exceed that during epidemic conditions. 45

Causes of the Condition

  • Influenza A (typically 70%-98%).
  • Influenza B (typically 2%-30%).
  • Balance between influenza A and B varies from year to year. During 2006/2007 season, 79% were influenza A and 21% were influenza B. 45


  • Influenza is caused by single-stranded RNA viruses of the family Orthomyxoviridae.
  • Hemagglutinin and neuraminidase are surface proteins important in diagnosis and treatment. The subtype is used to describe the virus, that is, H5N1 (avian flu) is hemagglutinin 5 and neuraminidase 1. The so-called "swine flu" is H1N1. Avian flu is discussed in more detail in a separate chapter.
  • Influenza A typically causes a more severe clinical illness than influenza B.
  • Persons are infectious from just before the onset of symptoms until 5 to 10 days after the onset of symptoms. Shedding is prolonged in children.