This webpage contains archival material about 2009 H1N1 Influenza A.
For current influenza information click here

Novel H1N1 Influenza A - A Primer

A novel strain of influenza A (Novel H1N1) is causing influenza illness worldwide. Many questions exist in regards the illness caused by this strain of influenza, treatment and prophylaxis, vaccination, and infection prevention.

Information below is adapted primarily from information provided by the Centers for Disease Control (CDC) but also from other authoritative organizations including the Infectious Diseases Society of America (IDSA), the Society for Healthcare Epidemiology of America (SHEA), and The Association for Professionals in Infection Control (APIC). Some guidance is adapted from local circumstances.

Regardless of the source no guideline can fit every clinical circumstance. It is crucial that clinicians exercise informed clinical judgment in the approach to persons ill with suspected or diagnosed influenza.

  • Epidemiology
  • Illness/Diagnosis
  • Treatment/Prophylaxis
  • Vaccine
  • Infection Prevention

Epidemiology of Novel H1N1 Influenza A

The epidemiology of novel H1N1 influenza A differs dramatically from that of typical seasonal influenza in that serious illness, hospitalization, and death is much less common in the elderly, more common in younger age groups. These data for the epidemiology for novel H1N1 influenza A were published in the MMWR (August 28, 2009 / Vol. 58 / No. RR--10):
  • Median age of laboratory confirmed infection with H1N1 influenza – 12 years
  • Median age of hospitalization for H1N1 influenza – 20 years
  • Median age of death due to H1N1 influenza – 37 years
  • As of July 2009 only 282 (5%) of 5,514 hospitalizations and 29 (8%) of the 353 reported deaths due to novel H1N1 influenza A had occurred among persons aged ≥65 years (CDC, unpublished data).

In typical flu seasons approximately 60% of hospitalizations and 90% of influenza related deaths occur in persons greater than 65 years of age. The graphic below illustrates the different epidemiology of novel H1N1 influenza A versus typical seasonal influenza.

Epidemiology H1N1 Influenza


Clinical Illness Due to Novel H1N1 Influenza A

  • Typical influenza signs and symptoms – fever, cough, sore throat, myalgias, headache, prostration
  • CDC case definition - Fever (temperature of 100°F [37.8°C] or greater) and a cough and/or a sore throat in the  absence of a known cause other than influenza
  • Some reports have emphasized increased gastrointestinal symptoms.
  • Fever is not invariably present.
  • Complications have included post-influenza bacterial pneumonias - Streptococcus pneumoniae, Staphylococcus aureus (including MRSA).
  • Roughly 2/3 of severe illness and death has occurred among those with traditional risk factors (therefore 1/3 of severe illness and death has occurred in persons lacking these risk factors). The American Academy of Pediatrics has published additional guidance in regards children considered at particular risk.
  • Severe influenza viral pneumonia in young healthy individuals has occurred.

How is Novel H1N1 Influenza A illness diagnosed?

  • Point of care nasopharyngeal swab rapid tests have only a 10-70% sensitivity for detecting infection with novel H1N1 influenza A.
    • Rapid tests can distinguish influenza A from influenza B.
    • Rapid tests don’t distinguish seasonal vs. novel H1N1 influenza A.
    • A negative rapid test does not rule out novel influenza A.
  • H1N1 PCR performed at Oregon State Public Health Laboratory
    • Only available for hospitalized patients who have had positive rapid influenza tests
    • Definitive but not timely
  • Outpatient diagnosis will depend upon clinical likelihood and current epidemiology.

Who Should be Treated for Novel H1N1 Influenza A?

  • Treatment is recommended for all hospitalized patients with confirmed, probable or suspected 2009 H1N1 or seasonal influenza.
  • Early empiric treatment should be considered for outpatients ill with influenza like illness who are at higher risk for influenza-related complications if initiated with first 48 hrs.
  • Signs and symptoms of severe illness due to suspected influenza (respiratory distress, confusion, severe dehydration) are an indication for immediate treatment, regardless of previous health or age. Infants and young children may manifest severe disease in a variety of ways.
  • Because of limitations of diagnostic tests treatment decisions usually will have to be made empirically.

Click here for treatment/prophylaxis regimens. An FDA Emergency Use Authorization (EUA) allows treatment with oseltamivir for children < 1 year of age. An additional FDA EUA has been issued for the use of peramivir, an intravenous antiviral agent that may be effective in the treatment of influenza.

When should prophylaxis be considered for persons exposed to persons ill with confirmed of likely influenza?

  • Consider for:
    • Persons who are at higher risk for complications of influenza and are a close contact of a person with confirmed, probable, or suspected novel H1N1 influenza during that person’s infectious period.
    • Health care personnel, public health workers, or first responders who have had a recognized, unprotected close contact exposure to a person with confirmed, probable, or suspected novel H1N1 influenza during that person’s infectious period if they are at higher risk of complications of influenza.
  • Antiviral agents should not be used for post exposure chemoprophylaxis in healthy children or adults based on potential exposures in the community, school, camp or other settings. Chemoprophylaxis generally is not recommended if more than 48 hours have elapsed since the last contact with an infectious person.
  • Early treatment is an emphasized alternative to chemoprophylaxis after a suspected exposure. Household or close contacts (with risk factors for influenza complications) of confirmed or suspected cases can be counseled about the early signs and symptoms of influenza, and advised to immediately contact their health care provider for evaluation and possible early treatment if clinical signs or symptoms of influenza develop.

How might treatment/prophylaxis recommendations be different if seasonal influenza should begin to co-circulate with novel H1N1 influenza?

  • Cases of seasonal influenza may be secondary to seasonal H1N1, H3N2, or influenza B viruses. These various strains of influenza vary in antiviral susceptibility such that effective therapy may require different choices of antiviral agents.
  • Knowledge of what strains of influenza are circulating will depend upon information provided by public health authorities.
  • Click here for a table describing antiviral susceptibility patterns depending upon what strains of influenza are circulating.


Vaccination is the preferred method of preventing all strains of influenza including Novel H1N1 Influenza A.

Vaccination for Seasonal Influenza

  • Trivalent vaccine - effective against seasonal H1N1 and H3N2 influenza A as well as influenza B
  • Safe and effective in preventing influenza illness/complications
  • May have even more benefit this year because of the complicated nature of this flu season with potential circulation of multiple strains of influenza—
    • Decreased transmission of infection
    • Decreased employee illness and absenteeism
    • Decreased demands on limited quantities of antiviral medications
  • Healthcare workers who decline vaccination may be required to wear surgical mask when providing patient care during the flu season.

Novel H1N1 Influenza A Vaccine

  • Monovalent (effective against Novel H1N1 Influenza A only)
  • Manufactured by same methods as trivalent seasonal influenza vaccine
  • No unusual safety issues identified in licensure trials
  • Available as inactivated intramuscular injectable and live attenuated intranasal administrable vaccines
  • Contraindicated in persons with severe egg allergy; history of Guillain Barre within 6 weeks of previous flu vaccine and moderate or severe acute illness are relative contraindications (intranasal live vaccine has additional contraindications)
  • Provided by U.S. government with distribution via local public health departments - being made available incrementally
  • Two doses necessary for children < 10 years

ACIP Recommended Use of Influenza A (H1N1) 2009 Monovalent Vaccine

  • Initial Target Groups (order of target groups does not indicate priority)
    • Pregnant women
    • Persons who live with or provide care for infants aged <6 months (e.g., parents, siblings, and daycare providers)
    • Health-care and emergency medical services personnel
    • Persons aged 6 months--24 years
    • Persons aged 25--64 years who have medical conditions that put them at higher risk for influenza-related complications.
  • Among these priority groups If there are initial shortages of vaccine, the ACIP suggests deferral of vaccination of healthy persons 5-24 years of age and those 19-64 years of age with high risk medical conditions.
  • Ultimately there will be enough vaccine for anyone who desires vaccination. In mid-December 2009 the Oregon State Health Dept indeed recommended that H1N1 vaccine be made available to anyone older than six months of age.

Vaccine Adverse Reactions/Contraindications

  • Injectable inactivated influenza vaccine
    • Adverse Reactions
      • Local reactions - 15%-20%
      • Fever, malaise - not common
      • Allergic reactions - rare
      • Neurological - very rare reactions
    • Contraindication - severe egg allergy; previous Guillain Barre Syndrome within 6 weeks of previous influenza vaccine and moderate or severe acute illness are relative contraindications.
  • Live Attenuated Influenza Vaccine (LAIV) - Contraindications
    • Children younger than 2 years of age
    • Persons 50 years of age or older
    • Persons with chronic medical conditions
    • Children and adolescents receiving long-term aspirin therapy
    • Immunosuppression from any cause
    • Pregnant women
    • Severe (anaphylactic) allergy to egg or other vaccine components
    • History of Guillain-Barré syndrome within 6 weeks of previous influenza vaccine is a relative contraindication.
    • Children younger than 5 years with recurrent wheezing
    • Moderate or severe acute illness is a relative contraindication.
  • Live Attenuated Influenza Vaccine - Adverse Reactions
    • Children
      • no significant increase in URI symptoms, fever, or other systemic symptoms
      • significantly increased risk of asthma or reactive airways disease in children 12-59 months of age
    • Adults
      • significantly increased rate of cough, runny nose, nasal congestion, sore throat, and chills reported among vaccine recipients
      • no increase in the occurrence of fever
    • No serious adverse reactions identified

Healthcare Workers and Live Attenuated Influenza Vaccine (LAIV)

  • Unless healthcare workers are caring for patients on hematopoietic stem cell transplant units where patients are in specialized "reverse" isolation they can receive LAIV without the need for special precautions.
  • LAIV transmission from a recently vaccinated person causing clinically important illness in an immunocompromised contact has not been reported.

Simultaneous Administration of LAIV and Other Vaccines

  • Inactivated vaccines can be administered either simultaneously or at any time before or after LAIV.
  • Seasonal and novel H1N1 LAIV should not be administered on the same day.
  • Other live vaccines can be administered on the same day as LAIV.
  • Live vaccines not administered on the same day should be administered at least 4 weeks apart.

Infection Prevention - General

  • Hand hygiene and respiratory etiquette
    • Signage with instructions prominently displayed
    • Hygiene/respiratory etiquette “stations”
  • Segregation of patients with influenza like illness (ILI) in waiting rooms, ERs, urgent cares
  • Surgical masks for patients suffering with ILI and healthcare workers evaluating them

Novel H1N1 Influenza Infection Prevention - Hospital

  • All hospitalized patients with influenza like illness should be placed in contact/droplet isolation. Any healthcare workers (HCWs) entering the room should use personal protective equipment (PPE):
    • Gowns, gloves, surgical masks
    • Eye protection if anticipating contact within 3 feet
  • HCWs should use N95 respirators if involved in procedures likely to cause aerosolization – intubation, bronchoscopy, open airway suctioning, autopsy

Hospital Isolation for Influenza

  • Isolation for confirmed or suspected influenza should be maintained for seven days from symptom onset or until symptoms abate whichever is longer.
  • Isolation can be discontinued if a rapid test for influenza is negative and influenza is judged unlikely by the responsible physician.
  • It is not necessary to have a negative H1N1 PCR result prior to discontinuing isolation if a physician judges infection unlikely.

Healthcare Worker Illness

  • Healthcare workers with suspected or confirmed influenza should be excluded from work for at least 24 hours after they no longer have a fever without the use of fever-reducing medicines – avoid “presenteeism.”
  • Healthcare workers returning to work after influenza like illness who work in areas where severely immunocompromised patients are provided care should be considered for temporary reassignment or exclusion from work for 7 days from symptom onset or until the resolution of symptoms, whichever is longer.