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Overview

Overview of Influenza

Influenza (flu) A & B are members of the virus family Orthomyxoviridae and are distinguished by the proteins that make up their viral coatings. Flu A subtypes include the 2009 H1N1 swine flu and H5N1 bird flu.

While both influenza A and B are capable of causing disease in humans, flu A is more common (71% of diagnosed cases)1, more virulent, and responsible for most of the flu pandemics2:

  • Flu A has animal as well as human hosts
  • Flu A mutates too rapidly for the immune system to create antibodies against current strains3
  • Flu B has only a human host
  • Flu B mutates slowly and humans have the time to build immunity4

The yearly trivalent flu vaccine protects against two strains of influenza A and one strain of influenza B.

Viral subspecies

Flu subspecies are defined by the proteins on their coating. The “H” stands for hemagglutinin, the “N” for the neuraminidase. There are 16 versions of hemagglutinin and 9 versions of neuraminidase. For example, 2009 H1N1 flu, contains hemagglutinin 1 and neuraminidase 1.

Flu infection

Once flu virus invades the airway, there is an incubation period of 18-72 hours. The virus causes cellular dysfunction and degeneration as it reproduces. Most patients recover from seasonal flu without long-term effects. However, various versions of flu A are capable of causing serious disease and death even in otherwise healthy persons.5

Because of viral shedding, adults infected with flu can be contagious 1 day before symptoms begin and for 5-7 days after the onset of illness. Children can remain contagious for even longer.6

Epidemiology of influenza

Influenza is one of the most common infections worldwide. According to the Centers for Disease Control & Prevention (CDC), on average each year in the United States7:

  • 5% to 20% of the population gets the flu
  • More than 200,000 people are hospitalized due to complications of flu
  • About 36,000 people die from flu-related causes
Influenza infection follows predictable seasonal patterns that have remained consistent since the CDC began surveillance. In the U.S., winter is the season for infection and the peak month for infection is Februrary.8




Selected Articles

Link arrow Transmission of influenza: implications for control in health care settings. Clin Infect Dis.
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Link arrow Severe morbidity and mortality associated with influenza in children and young adults—Michigan, 2003. MMWR.
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Link arrow 2009-2010 Influenza season triage algorithm for adults (older than 18 years of age) with influenza-like illness. CDC.
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