Brief Report

When Pneumonia Occurs with Flu: Think Influenza

By Carol A. Kemper, MD, FACP

Clinical Associate Professor of Medicine, Stanford University, Division of Infectious Diseases, Santa Clara Valley Medical Center

Dr. Kemper does research for Abbott Laboratories and Merck. This article originally appeared in the August issue of Infectious Disease Alert.

Source: Jain S, et al. Influenza-associated pneumonia among hospitalized patients with 2009 pandemic influenza A (H1N1) virus — United States, 2009. Clin Infect Dis 2012;54:1221-1229.

CASES OF PANDEMIC H1N1 REQUIRING HOSPITALIZATION were examined by reviewing two national case series from spring and fall 2009. During this period, a total of 451 patients with laboratory-confirmed H1N1 were hospitalized, 195 (43%) of whom were diagnosed with pneumonia based on chest radiographs. Not unexpectedly, those patients with pneumonia had higher rates of admission to the intensive care unit (52% vs 16%), and were more likely to be diagnosed with acute respiratory distress syndrome (26% vs 2%), sepsis (18% vs 3%), and mortality (17% vs 2%), than those without pneumonia. More than half of those with pneumonia had bilateral infiltrates (67%); the others had multilobar infiltrates (7%) or unilobar involvement (31%). Bacterial infection, mostly bacteremia, was confirmed in 13 patients (7%) with pneumonia and 2 (< 1%) of those without.

What was not necessarily expected was the finding that patients with influenza-associated pneumonia were less likely to receive antivirals within 48 hours of admission compared with those admitted with influenza without pneumonia (28% vs 50%, P < 0.0001). Eventually during the hospitalization, a similar proportion of patients with or without influenza-associated pneumonia did receive antiviral therapy (78% vs 79%); 91% of this was oseltamivir.

The key to this paradox may be that the very presence of pneumonia or infiltrates on chest radiographs was more likely to prompt a diagnosis of bacterial infection and administration of antibacterials, rather than trigger a suspected diagnosis of influenza. “Sepsis” (which was based on clinical judgment) was diagnosed in 18% of these pneumonia cases, compared with only 2% of non-pneumonia cases, suggesting either bias in the suspicion of bacterial infection — or even more possibly, a more severe systemic inflammatory response from H1N1 infection in those with pneumonia.

During influenza season, influenza (H1N1) should be included in the differential of patients admitted with severe illness and pneumonia, or “sepsis” and pneumonia, and presumptive antiviral treatment should be started as soon as possible, at least until additional information and the results of tests are available.