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ContentsSynopsisCodesLook ForDiagnostic PearlsDifferential Diagnosis & PitfallsBest TestsManagement PearlsTherapyReferencesInformation for PatientsView all Images (4)
Influenza A H1N1 - Pulmonary
Other Resources UpToDate PubMed

Influenza A H1N1 - Pulmonary

Print Patient Handout Images (4)
Contributors: Ghinwa Dumyati MD, Susan Voci MD, Sumanth Rajagopal MD, William Bonnez MD
Other Resources UpToDate PubMed


Current Flu Season
Per the Centers for Disease Control and Prevention (CDC), the 2017–2018 influenza season was high severity, with influenza A (H3N2) viruses predominating. Peak clinical laboratory influenza positivity ranged from the end of December to mid-February. Influenza activity in the United States is expected to pick up in Fall 2018. See Influenza.

2009 H1N1 Pandemic
In April 2009, the CDC identified in US patients a novel influenza A H1N1 virus characterized by a unique combination of gene segments not previously seen. Also referred to as pandemic H1N1 2009 influenza virus, novel swine-origin influenza A (H1N1) virus (S-OIV), H1N1 flu, and swine flu, it is a quadruple reassortant virus containing genes from 2 swine influenza viruses known to circulate in pigs in North America and Eurasia as well as genes from avian and human influenza viruses. Unlike previously documented cases of swine influenza A/H1N1 infection in humans, the 2009 virus spread easily among humans with no pig exposure. It is now considered a regular human flu virus and continues to circulate seasonally worldwide.

The most common mild symptoms are fever, cough, and sore throat, with a quarter of patients experiencing diarrhea and/or vomiting. However, the clinical spectrum does include severe outcomes including respiratory failure and death. Many patients have fever, cough, dyspnea or respiratory distress, increased serum lactate dehydrogenase levels, bilateral patchy pneumonia, increased creatine kinase level, and lymphopenia. Requirement for mechanical ventilation and death can occur.

Patients at highest risk for severe illness and complications are children aged younger than 5 years, children and adults of any age with underlying chronic medical conditions, and pregnant women. Of note, rates of hospitalization and morbidity have been proportionately lower for people aged older than 60, perhaps because those born before 1957 had previous exposure to influenza A H1N1 strains, which disappeared from circulation for about 20 years after the 1957 influenza A H2N2 pandemic. Even so, adults aged 65 and older are currently defined as being at high risk for complications.

Secondary complications likely include primary viral pneumonia and secondary bacterial pneumonia (specifically, group A streptococci, Staphylococcus aureus, and Streptococcus pneumoniae), including ventilator-associated pneumonia (eg, from Acinetobacter baumannii, Achromobacter xylosoxidans, methicillin-resistant Staphylococcus aureus [MRSA], or Escherichia coli).

H1N1 outbreaks have differed from those of seasonal influenza in the following ways:
  • Atypical timing (seasonal influenza is usually observed from October through mid-May in the United States).
  • A shift in the age groups most severely affected to a younger population.
  • The percentage of deaths is highest (39%) among people aged 25-49 years, followed by people aged 50-64 years (25%). (Seasonal influenza estimated percentage is 90% in people aged older than 65.)
Transmission among humans is thought to occur via aerosol droplets expelled when an infected person coughs or sneezes. It is possible, too, that novel influenza A H1N1 can be spread through contact with fomites containing respiratory or gastrointestinal material. Also, because about a quarter of patients experience diarrhea, the potential exists for fecal viral shedding and subsequent fecal-oral transmission. Until more data are acquired, all potential routes of transmission should be considered.

The incubation period for novel influenza A H1N1 infection appears to be 2-7 days, with patients likely contagious 1 day prior through 7 days after the onset of symptoms or until symptoms resolve. An annual vaccine is available for seasonal influenza and includes protection against the influenza A H1N1 virus.

Pediatric Patient Considerations
The infectious period could be longer in young children. As with seasonal influenza, children aged younger than 5 years are at higher risk for severe complications.

Immunocompromised Patient Considerations
The infectious period could be longer in immunocompromised or severely ill patients. As with seasonal influenza, immunocompromised patients – including pregnant women – are among those at highest risk for severe complications.


J09.X2 – Influenza due to identified novel influenza A virus with other respiratory manifestations

442696006 – Influenza due to Influenza A virus subtype H1N1

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Differential Diagnosis & Pitfalls

The differential includes other viral respiratory diseases that present with ILI.
Seasonal and local occurrences of influenza should be considered. Co-circulation of other influenza viruses (eg, human influenza A [H1N1] virus, human influenza A [H3N2] virus, or influenza B virus) with novel H1N1 influenza A will make diagnosis more challenging.

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Last Updated: 09/14/2018
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Influenza A H1N1 - Pulmonary
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Influenza A H1N1 : Cough, Fever, Headache, Dyspnea, Myalgia, Pharyngitis, LYP decreased
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