Through November, 2009, approximately 99% of typed influenza viruses have been 2009 H1N1. The vast majority of 2009 H1N1 viruses tested for drug resistance have been susceptible to oseltamivir and zanamivir but resistant to the adamantanes (amantadine, rimantadine). Surveillance data, updated weekly, are available at www.cdc.gov/flu/weekly These recommendations will be revised as needed to adapt to new information on risk factors, antiviral availability and resistance, or the circulation of other influenza viruses.

In general, treatment with an antiviral agent, when indicated, should begin as soon as possible after the onset of typical influenza-like symptoms. Influenza illness can present in a range of symptomatology: from a mild upper respiratory infection to an acute, life-threatening illness.

• Mild or uncomplicated illness is characterized by typical symptoms like fever (although not everyone with influenza will have a fever), cough, sore throat, rhinorrhea, muscle pain, headache, chills, malaise, sometimes diarrhea and vomiting, but no shortness of breath and little change in chronic health conditions.
• Progressive illness is characterized by typical symptoms plus signs or symptoms suggesting more than mild illness: chest pain, poor oxygenation (e.g. tachypnea, hypoxia, labored breathing in children), cardiopulmonary insufficiency (e.g. low blood pressure), CNS impairment (e.g. confusion, altered mental status), severe dehydration, or exacerbations of chronic conditions (e.g. asthma, chronic obstructive pulmonary disease, chronic renal failure, diabetes or other cardiovascular conditions).
• Severe or complicated illness is characterized by signs of lower respiratory tract disease (e.g., hypoxia requiring supplemental oxygen, abnormal chest radiograph, mechanical ventilation), CNS findings (encephalitis, encephalopathy), complications of low blood pressure (shock, organ failure), myocarditis or rhabdomyolisis, or invasive secondary bacterial infection based on laboratory testing or clinical signs (e.g. persistent high fever and other symptoms beyond three days).

Influenza viruses are transmitted from person to person primarily through contact with infected respiratory secretions, especially airborne droplets generated by coughing and sneezing. Viral replication and shedding are key considerations in the timing of treatment, infection control, and chemoprophylaxis.

In general, the incubation period for influenza is estimated to range from 1 to 4 days with an average of 2 days. Influenza virus shedding (the time during which a person might be infectious to another person) begins the day before illness onset and can persist for 5 to 7 days, although some persons may shed virus for longer periods, particularly young children and severely immunocompromised persons. The amount of virus shed is greatest in the first 2-3 days of illness and appears to correlate with fever, with higher amounts of virus shed when temperatures are highest. For these recommendations, however, the infectious period for influenza is defined as one day before fever begins until 24 hours after fever ends.

Treatment of Confirmed or Suspected Influenza Who to treat Prompt empiric treatment is recommended for persons with suspected or confirmed influenza and:

• Illness requiring hospitalization
• Progressive, severe, or complicated illness, regardless of previous health status, and/or
• Patients at risk for severe disease

How to treat

• Antiviral drugs: oseltamivir (oral), zanamivir (inhaled)
• Initiate treatment as early as possible after onset of symptoms
• Treat empirically before diagnostic test results are reported
• When definitive diagnosis is indicated, request definitive diagnostic tests rather than rapid tests