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The Role of Bacterial Pneumonia in Pandemic Influenza

By Eric Toner, M.D., September 12, 2008

A paper1 in the October issue of the Journal of Infectious Diseases by Morens, Taubenberger, and Fauci from the National Institute of Allergy and Infectious Diseases concludes that, contrary to what some scholars of the 1918 pandemic had believed, most deaths during the 1918 flu pandemic appear to have involved bacterial pneumonia rather than purely viral pneumonia. This finding contrasts with data from current human cases of H5N1 that manifest as purely viral pneumonia, for reasons that are not clear. This new study by Morens and colleagues is important because the role that bacterial suprainfections may play in causing illness and death from pneumonia during an influenza pandemic has been under appreciated in most pandemic planning, and it has important implications for flu planners as they project likely health consequences, identify medical surge capacity needs, and stockpile antimicrobials.   

With seasonal influenza, pneumonia is the usual reason for both hospitalization and death. A minority of cases are caused by purely viral infections, but most are due to combined viral and bacterial infections or are the result of secondary bacterial infection due to impaired host defenses. Historical accounts of the 1918 pandemic include many cases of fulminant pneumonia, with clinical descriptions suggesting viral pneumonia. However, this study by Morens and colleagues is the first to report large numbers of contemporaneous culture results and to examine tissue specimens from victims. 

The researchers reviewed the pathologic and bacteriologic results of 109 autopsy series representing 8398 cases from the 1918 pandemic. They also examined actual lung tissue specimens from 58 autopsies of American soldiers who died of the flu in 1918-1919. The specimens had been saved in the National Tissue Repository of the Armed Forces Institute of Pathology. The researchers found that nearly all the tissue specimens had pathological features consistent with acute bacterial pneumonia, either alone or in conjunction with evidence of viral pneumonia. 

Of 68 high-quality autopsy series in which post mortem bacteriologic cultures of the lung were performed, bacterial growth was observed in 93% of the cases. The most common isolates were Streptococcus pneumoniae and Streptococcus pyogenes or mixed cultures of pneumopathogens. Staphylococcus aureus and Haemophilus influenzae were found less commonly. Cultures of blood and pleural fluid provided similar results (see table below). 

Table: Post mortem bacterial culture results

SpecimenNo. of series (cases)S. pneumoniaeS. pyogenesMixed or otherNo growth

68 (3074)






42 (1887)






35 (1245)





These results provide strong evidence that many, if not most, of the pneumonia deaths during the 1918 pandemic involved bacterial infections. This suggests that many patients might have survived the 1918 pandemic had antibiotics been available, and suggests as well that if a future flu pandemic is similar to that of 1918, most patients with pneumonia will require antibiotics in addition to antivirals. Pandemic planners should therefore be assuring an adequate supply of antibiotics as well as antivirals.


  1. Morens DM, Taubenberger JK, and Fauci AS. Predominant Role of Bacterial Pneumonia as a Cause of Death in Pandemic Influenza: Implications for Pandemic Influenza Preparedness. J Inf Dis 2008;198:962–970

  2. Korteweg C and Gu J. Pathology, Molecular Biology, and Pathogenesis of Avian Influenza A (H5N1) Infection in Humans. Am J Pathol 2008 172: 1155-1170.