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Clinical Aspects of H1N1 Influenza A Infection

By Amesh A. Adalja, MD, October 14, 2009

Several research reports detailing aspects of infection with the pandemic strain of influenza have recently been released. These reports describing clinically relevant aspects of cases are useful for day-to-day management of pandemic influenza.

Details on Hospitalization of U.S. Patients

The New England Journal of Medicine's summary1 of data on U.S. patients hospitalized with H1N11 includes the following important findings:

  • 25% of hospitalized patients required intensive care, 65% of whom required mechanical ventilation

  • 40% of patients undergoing chest radiography had evidence of pneumonia

  • 75% of patients were treated with antiviral medications

  • 79% of patients received antibiotics, mostly before or at admission

  • 93% of patients were discharged; 7% of patients died

  • Multivariate analysis revealed that those who received antiviral therapy within 2 days of symptom onset were significantly more likely to experience a positive outcome.

Bacterial Co-infection Seen in Less than 30% of Fatal Cases

Like seasonal influenza and past pandemics, a significant percentage of influenza related pneumonia involved bacterial co-infection, according to the October 2, 2009, MMWR.2 Of 77 fatal cases studied by the CDC, 22 (29%) displayed evidence of bacterial co-infection at autopsy. Several organisms were isolated (see table below); pneumococcus was the most prevalent, underscoring the potential importance of both the Pneumovax and Prevnar vaccines. In many of these cases, bacterial co-infection was not documented ante mortem, highlighting the known insensitivity of clinical cultures. Because of sampling bias, this study may overestimate the true incidence of bacterial co-infection.

PathogenPercentage of
Cases of
Streptococcus pneumoniae (pneumococcus)45%
Staphylococcus aureus32%
Streptococcus pyogenes (Group A streptococcus) 27%
Streptococcus mitis (viridians group streptococci) 9%
Haemophilus influenzae4.5%
Multiple pathogens18%

Interesting Clinical Presentations

With the accumulation of case data, unusual presentations of influenza are also being discovered, such as an IgG subclass deficiency in many patients with severe disease identified by Lindsay Grayson and colleagues and revealed in San Francisco at the 2009 ICAAC meeting. This finding raises the question of potential benefit from IVIG infusion.3 Another interesting presentation, which occurred in Canada and was reported in Lancet, was that of a hemodialysis dependent patient who presentied with hypervolemia and no fever. Influenza was not suspected until 4 days after admission.4

Understanding of Influenza Disease Has Expanded Substantially

As such reports accumulate, they will help paint a more detailed picture of severe influenza and the spectrum of disease to be expected during a pandemic. And each report provides more evidence for clinicians to use in managing H1N1 influenza (A) in individual patients, and for policymakers apply in managing this pandemic for the nation.


  1. Jain S, Kamimoto L, Bramley AM, et al. Hospitalized patients with 2009 H1N1 influenza in the United States, April–June 2009. NEJM 2009. Accessed October 12, 2009.

  2. CDC. Bacterial coinfections in lung tissue specimens from fatal cases of 2009 pandemic influenza A (H1N1)—United States, May-August 2009. MMWR 2009. 58:1071-1074. Accessed October 12, 2009.

  3. ProMED-mail. Influenza pandemic (H1N1) 2009 (51): antibody deficiency. September 17, 2009.,F2400_P1001_
    . Accessed October 12, 2009.

  4. Wiebe C, Reslerova M, Komenda P, et al. Atypical clinical presentation of H1N1 influenza in a dialysis patient. Lancet 2009. 374:1300.

Editor’s Note: On Friday, October 16, we will publish a second CBN Report summarizing findings from several studies of critically ill patients from around the world.