NOTE: This article was current when written on May 20, 2009, but information (including management guidelines) has changed daily. See the DynaMed Web site for current information.


• Concern for risk of “imminent pandemic” 
• Newly identified influenza virus A/California/04/2009 A (H1N1) contains genetic components from human, swine, and avian strains.
• Human-to-human transmission by exposure to respiratory droplets or contaminated surfaces; not transmitted by eating pork

Also called

• Swine influenza; swine flu
• Swine-origin influenza virus (S-OIV)
• H1N1 influenza; H1N1 flu
• Novel H1N1 influenza

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ICD-9 codes

• 487.0 influenza with pneumonia
• 487.1 influenza with other respiratory manifestations
• 487.8 influenza with other manifestations

CDC case definitions

• Influenzalike illness (ILI)—fever plus sore throat or cough
• Confirmed case—person with ILI with novel influenza A (H1N1) virus infection CDC laboratory confirmed by real-time reverse transcriptase polymerase chain reaction (RT-PCR) or viral culture
• Probable case—person with ILI who is positive for influenza A but negative for H1 and H3 by influenza real-time RT-PCR
• Suspected case—person who 
    — Does not meet confirmed or probable case definition 
    — Is not novel H1N1 test-negative
    — Has ILI in one of following situations 

    • Previously healthy person <65 years old hospitalized for ILI 
    • Travel to state or country with one or more confirmed or probable cases 
    • Epidemiologic link in past seven days to confirmed or probable case


• Similar to those of seasonal influenza, including fever, cough, sore throat, rhinorrhea, nasal congestion, lethargy, lack of appetite, nausea, vomiting, diarrhea
• Symptoms of severe disease in children may include apnea, tachypnea, dyspnea, cyanosis,dehydration, altered mental status, extreme irritability.

Differential diagnosis

• Influenza (non-avian, non-swine)
• Avian influenza
• Other viral flulike syndrome—difficult to distinguish clinically
• Carbon monoxide poisoning
• Bacterial respiratory infection

Diagnostic testing

• Check local public health recommendations for patient selection for testing and use of rapid antigen tests.
• Interpretation and management of rapid test results vary by community and timing of outbreak.
• Respiratory swab generally needed in first four to five days of illness for viral detection, but some persons (especially children) may shed virus for >10 days
• Send nasopharyngeal swab or nasal aspirate to appropriate public health laboratory for real-time RT-PCR. 
    — Nasal swab or oropharyngeal swab is acceptable (but not preferred) for specimen collection. 
    — Swab should be synthetic tip on plastic or aluminum shaft.
    — Place in 1-3 mL of sterile viral transport medium. 
    — Store in refrigerator until shipped on dry ice in appropriate packaging.


• Among 10,243 cases reported worldwide (as of May 20, 2009), 80 deaths have occurred (0.8% case fatality rate).
• Most confirmed cases in United States have been mild.
• Individuals at high risk for complications include patients with chronic medical conditions or immunosuppression, patients <5 or >50 years old, children and adolescents taking chronic aspirin therapy (risk for Reye syndrome), pregnant women, patients in chronic-care facilities.


• Recommendations may change as data on antiviral susceptibilities become available. 
    — Antiviral resistance testing for U.S. cases (April 2009): 100% susceptible to oseltamivir (Tamiflu) or zanamivir (Relenza) and 100% resistant to amantadine and rimantadine
    — Seasonal influenza antiviral sensitivities vary with subtype.
• Antiviral treatment for five days 
    — Oseltamivir 75 mg orally b.i.d. (lower dose for patients 0-13 years old) 
    — Zanamivir 10 mg (two inhalations) via inhaler b.i.d. for patients >7 years old
    — If H1N1 or seasonal influenza possible, use zanamivir or combine oseltamivir plus amantadine or rimantadine.
• Supportive measures: antipyretics, oral fluids, nutrition, bed rest
• Avoid aspirin in children <18 years old (due to risk of Reye syndrome).
• Preventive measures for patients, caregivers, close contacts, and health-care professionals
    — Antiviral chemoprophylaxis for high-risk close contacts 
    — Social distancing, respiratory etiquette, face masks


• No vaccine available
• General precautions: frequent hand-washing,covering coughs and sneezes, advising ill persons to stay home (except to seek medical care) and minimize contact with others in household, voluntary home quarantine of household contacts, reduction of unnecessary social contacts
• Place patients with confirmed, probable, or suspected case in single-patient room with door kept closed. 
    — Limit entry to those providing direct patient care. 
    — Gown, gloves, eye protection and N95 respirator for persons providing care
    — Ill patient should wear surgical mask when outside room and wash hands frequently.
• Antiviral chemoprophylaxis
    — Oseltamivir or zanamivir once daily during exposure period and for 10 days after last known exposure
    — Indicated for

  • Close contacts (of confirmed, probable, or suspected case) at high-risk for complications of influenza
    • Chronic medical conditions 
    • Aged >65 years 
    • Aged <5 years 
    • Pregnant women 
  • Health-care workers, public health workers, or first responders who have recognized unprotected close contact during a case’s infectious period

For complete references, see

Dr. Alper is medical director of clinical reference products for EBSCO Publishing Inc., in Ipswich, Mass., and editor-in-chief of DynaMed, a database that contains a more comprehensive and continually updated summary of this and more than 3,000 other clinical topics.