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Clinical Details of 2009 H1N1 in Hospitalized Patients

By Amesh A. Adalja, MD, May 22, 2009

As the outbreak of the 2009 H1N1 influenza A virus continues, important clinical details about the more severely affected patients are beginning to emerge. These details of hospitalized patients are vital information for clinicians who may need to manage other patients. And this information can also serve to inform policymakers regarding what scenarios to expect and the resources that are likely to be needed.

California Hospitalizations Reveal Co-morbidities Are Common

As of May 17, 2009, 30 patients infected with 2009 H1N1 have been hospitalized in California. On May 18, 2009, the Centers for Disease Control and Prevention (CDC) released an MMWR detailing the clinical characteristics of those patients. The following table summarizes the demographic features of the cases.1

Median Age27.5 years
(27 days- 89 years)
Sex21 female, 9 male
Travel to Mexico in the 7 days prior to illness13%
Known exposure to another case0

Pneumonia and dehydration were the most common admission diagnoses. Underlying medical conditions were present in 64%, including chronic lung diseases, conditions requiring immunosuppression, cardiac disease, diabetes, and obesity. Common symptoms were fever, coughing, vomiting, and shortness of breath.1

Pneumonia Is a Common Radiographic Finding

Radiographic abnormalities consistent with pneumonia were found in 15 of 25 (60%) of those who underwent chest radiography—10 of whom had multilobular infiltrates. Six patients required admission to intensive care, and 4 patients required mechanical ventilation.1

Rapid Tests Relatively Unreliable

Diagnosis of influenza was made by rapid antigen testing, viral culture, and direct immunofluorescent antibody. Rapid tests were positive in 16 of 21 patients (76%) and negative in 5 (24% false negative rate).1

Bacterial Superinfection Not Found

No evidence of bacterial superinfection was found in any of the 30 patients. Endotracheal, bronchoalveolar lavage, blood, urine, and sputum cultures were negative in all case patients.1

Oseltamivir Employed in Only Half

Antiviral therapy in the form of oseltamivir was used in half of the patients. Some of those who did not receive antiviral therapy sought care after 48 hours of symptoms, the time window after which antiviral therapy is less effective. However, the current CDC guidelines do not recommend withholding antiviral therapy in hospitalized patients who present outside the 48-hour window.1

Disease in Pregnant Patients

The reports from California, as well as in a prior MMWR release, detail the course of illness in pregnant patients, a population that has sustained at least 1 death. The California report says that severe outcomes occurred in 2 patients. One suffered a premature rupture of the membranes (PROM), and another suffered a spontaneous abortion. In the prior report, the case histories of several pregnant women were reported, including 1 pregnant patient who died as a result of infection after developing Acute Respiratory Distress Syndrome (ARDS). She had first presented for care on April 15, 2009, but antiviral therapy was not initiated for 14 days.1,2

A Clinical Picture Emerges

The above findings begin to fill in the details of what to expect with more severe cases of influenza caused by the new strain. As the outbreak develops further, the early case reports will guide treatment decisions and resource allocation. The most striking feature of the current case reports is the fact that no evidence of bacterial superinfection was found in severely ill patients—a finding in contradistinction to some recent studies of the 1918 pandemic and more characteristic of infections with H5N1.3,4 This trend, if it continues, may have implications for antibiotic use as well as for any plans to expand vaccination for bacterial pathogens that have traditionally complicated influenza (eg, Hemophilus influenzae, Streptococcus pneumoniae).

Severe disease in pregnant patients is a hallmark of influenza; however, the facts surrounding the fatality underscore the imperative to initiate antiviral therapy in pregnant patients. Despite the fact that oseltamivir has a Category C rating from the Food and Drug Administration (FDA), CDC states, “Any potential risk to a fetus is likely outweighed by the expected benefits of influenza antiviral treatment for this novel virus.” CDC guidelines state that pregnant patients with confirmed, probable, or suspected cases of influenza should receive antiviral treatment.

The case histories of more patients, especially the fatal cases in Mexico, will help to complete the picture of the full spectrum of illness this novel virus can induce.


  1. U.S. Centers for Disease Control and Prevention. Hospitalized patients with novel influenza A (H1N1) virus infection—California, April-May, 2009. MMWR Morb Mortal Wkly Rep 2009;58 [early release]. Accessed May 20, 2009.

  2. U.S. Centers for Disease Control and Prevention. Novel influenza A (H1N1) virus infections in three pregnant women—United States, April-May, 2009. MMWR Morb Mortal Wkly Rep 2009;58:497-500. Accessed May 20, 2009.

  3. Morens DM, Taubenberger JK, Fauci AS. Predominant role of bacterial pneumonia as a cause of death in pandemic influenza: implications for pandemic influenza preparedness. J Infect Dis 2008;198:962-970. 

  4. Korteweg C, Gu J. Pathology, molecular biology, and pathogenesis of avian influenza A (H5N1) infection in humans. Am J Pathol 2008;172:1155-1170.