Historically and clinically, influenza is the most important viral respiratory disease of mankind, killing up to 500,000 people every year on a global basis.

RNA virus of the Orthomyxoviridae family

Three types: Influenza A/B/C; A and B are the most important pathogens in humans.

Virus consists of eight single strands of RNA contained in a lipoprotein envelope studded with two antigenic proteins:

Influenza A has an error-prone RNA polymerase, creating subtle changes in the nature of HA and N, resulting in antigenic drift. This is the main factor necessitating annual vaccination with the most current strains of virus.

Influenza A can also undergo reassortment of the RNA genome. This occurs when two different viruses simultaneously coinfect a host cell, creating a novel combination of HA and N. This is called antigenic shift and is the cause of pandemic spread of influenza.

There were three pandemics in the 20th century and thus far one in the 21st century. The 1918 pandemic resulted in 50 million deaths, most occurring disproportionately in healthy young adults.

Influenza virus can at times cause zoonotic infection. This was shown to occur recently when avian influenza (H5N1), also known as bird flu, crossed species boundaries and caused human infection. Only rarely was the virus subsequently able to be transmitted person to person.

The 2009 pandemic flu was a variant of seasonal H1N1. It was a quadruple reassortment of two swine strains, one human strain, and one avian strain.

Influenza B has far lesser propensity for antigenic changes, and only antigenic drifts in the HA have been described.

H3N2 and seasonal H1N1 are the two most common influenza A subtypes in circulation, in addition to influenza B.

Influenza causes an average of 200,000 hospitalizations and 41,000 deaths in a typical endemic year in the United States.^1,2

Elderly populations have the highest rate of hospitalization following influenza.¹

CDC in collaboration with WHO publishes weekly updates on the activity of influenza virus throughout the world, which is accessible at www.cdc.gov/flu/weekly.

The peak influenza activity in northern latitudes is in late fall and winter, with late January and early February being the peak season in the United States.

Infected individuals shed virus from 1 day prior and 5 to 6 days after the onset of symptoms³ with peak at day 2 to 3. Prolonged shedding occurs in hospitalized patients with severe disease and immunocompromised patients.^4,5

Influenza virus is transmitted both by aerosols induced by cough/sneeze and by contact with contaminated surfaces.

The virus is inactivated by sunlight, disinfectants, and detergents.

During an epidemic period, the presence of fever, cough, and illness duration <7 days has a sensitivity of 78% and specificity of 73% of being influenza.⁶ Other symptoms include sore throat, hoarseness, and myalgias.

Most cases of influenza are self-limited, though complications occur with increased frequency in high-risk populations.

High-risk populations include:

Some influenza strains have the capacity to induce an uncontrolled and highly destructive immune response, largely mediated by proinflammatory cytokines, resulting in ARDS and multisystem organ failure. This response to infection is capable of inducing high case fatality rates in healthy individuals aged 18 to 40.⁸

Complications include: