Washington, DC - When U.S. adults are hospitalized with pneumonia, viruses are more often to blame than bacteria.  However, despite current diagnostic tests, neither viruses nor bacteria are detected in the majority of these patients according to a study by the Centers for Disease Control and Prevention (CDC) released today in the New England Journal of Medicine.

This two-and-a-half year study conducted by researchers at CDC, three hospitals in Chicago, and two hospitals in Nashville estimated the burden of community-acquired pneumonia hospitalizations among U.S. adults.

“Pneumonia is a leading cause of hospitalization and death among adults in the United States and in 2011 the medical costs exceeded $10 billion,” said CDC Director Tom Frieden, M.D., M.P.H. “Most of the time doctors are unable to pinpoint a specific cause of pneumonia. We urgently need more sensitive, rapid tests to identify causes of pneumonia and to promote better treatment.”

The CDC Etiology of Pneumonia in the Community (EPIC) study was a prospective, multi-center, population-based study that used chest x-rays and extensive diagnostic methods to determine the incidence and etiology of community-acquired pneumonia hospitalizations among U.S. adults. Participants in the study were enrolled from January 2010 through June 2012 in three hospitals in Chicago and two in Nashville. Study participants provided specimens that were tested for viral and bacterial respiratory pathogens.

During the study, the EPIC team enrolled 2,488 eligible adults, of which 2,320 (93 percent) had radiographically confirmed pneumonia. The median age of study participants was 57 years.

The researchers detected viruses in 27 percent of patients and bacteria in 14 percent of patients. Human rhinovirus (HRV) was the most commonly detected virus among pneumonia patients.

Influenza (flu) was the second most common viral pathogen detected, and there were twice as many pneumonia hospitalizations due to influenza than any other viral pathogen (except HRV) in adults 80 years or older, underscoring the need for improvements in flu vaccine uptake and effectiveness.

Together, human metapneumovirus, respiratory syncytial virus, parainfluenza virus, coronavirus, and adenovirus were detected in 13 percent of patients.

Of bacterial pathogens, Streptococcus pneumoniae was the most commonly detected bacterium, causing an estimated five times more pneumonia hospitalizations in adults 65 years and older than in younger adults. Mycoplasma pneumoniae, Legionella pneumophila, and Chlamydophila pneumoniae combined were detected in 4 percent of patients. Overall, Staphylococcus aureus was detected in 2 percent of patients and was found less frequently than S. pneumoniae or viruses.

S. pneumoniae, S. aureus, and Enterobacteriaceae were significantly more common among severely ill patients, accounting for 16 percent of detections among intensive care unit (ICU) patients compared with 6 percent among non-ICU patients.

“The frequency with which respiratory viruses were detected in adults hospitalized with pneumonia was higher than previously documented. This may be due to improved molecular diagnostics for viruses and also to the benefits of bacterial vaccines,” said Dr. Seema Jain, lead author of the paper and medical epidemiologist in CDC’s Influenza Division. “However, what’s most remarkable is that despite how hard we looked for pathogens, no discernible pathogen was detected in 62 percent of adults hospitalized with pneumonia in the EPIC study. This illustrates the need for more sensitive diagnostic methods that can both help guide treatment at the individual level as well as inform public health policy for adult pneumonia at a population level.”

The CDC EPIC study is one of the largest population-based pneumonia studies ever conducted in the United States.