Viral infections appear to be the primary cause of pneumonia that results in hospitalization, according to a Centers for Disease Control and Prevention study of five urban hospitals in Chicago and Nashville, Tenn.

The study was published online July 14 in the New England Journal of Medicine.

From January 2010 through June 2012, 2,259 patients hospitalized for community-acquired pneumonia (CAP) had both radiographic evidence of disease and specimens for bacterial and viral testing. A pathogen was detected in just 853 (38%). One or more viruses were detected in 530 (23%), bacteria in 247 (11%), bacterial and viral pathogens in 59 (3%), and fungal or mycobacterial pathogens in 17 (1%). The findings indicate the annual incidence of community-acquired pneumonia requiring hospitalization is about 25 cases per 10,000 adults, with the highest rates among adults 65-79 years old (63 cases per 10,000) and 80 years or older (164 cases per 10,000).

The annual incidence of community-acquired pneumonia requiring hospitalization is about 25 cases per 10,000 adults, according to CDC researchers. Courtesy NIH National Library of Medicine/Institute for Physical Education and Sport, University of Malta, Malta/Creative Commons License

The annual incidence of community-acquired pneumonia requiring hospitalization is about 25 cases per 10,000 adults, according to CDC researchers.

The most common pathogens were rhinovirus in 9% of patients, influenza virus in 6%, and Streptococcus pneumoniae in 5%, which was the most commonly detected bacterium (N. Engl. J. Med. 14 July 2015 [doi:10.1056/NEJMoa1500245]).

CAP is thought to be caused most often by S. pneumoniae and other bacteria; the fact that viruses were more frequently detected “probably reflects the direct and indirect benefit of bacterial vaccines,” but also “relatively insensitive diagnostic tests,” said the investigators, led by CDC medical epidemiologist Dr. Seema Jain.

The project, dubbed the Etiology of Pneumonia in the Community (EPIC) study, “adds to the growing evidence of the contribution of viruses to hospitalizations of adults.” It also suggests “that improving the coverage and effectiveness of recommended influenza and pneumococcal vaccines and developing effective vaccines and treatments for human metapneumovirus, respiratory syncytial virus, and parainfluenza virus” – also found in study patients – “could reduce the burden of pneumonia among adults,” the researchers said.

The work matters because “the last U.S. population–based incidence estimates of hospitalization due to community-acquired pneumonia were made in the 1990s, before the availability of the pneumococcal conjugate vaccine and more sensitive molecular and antigen-based laboratory diagnostic tests. Thus, contemporary population-based etiologic studies involving U.S. adults with pneumonia are needed,” they noted.

Blood, urine, and respiratory cultures; serologic testing; antigen detection; molecular diagnostics, and chest x-rays were all used to find the cause of disease. The fact that infections were found in just 38% of the patients could have something to do with the team’s “inability to obtain lower respiratory tract specimens, antibiotic use before specimen collection, [and] insensitive diagnostic tests for known pathogens,” among other problems, they said.

There was a low prevalence of Enterobacteriaceae (1%) and other gram-negative bacteria, probably because patients with recent hospitalizations and severe immunosuppression were excluded. Subjects were on average 60 years old.

The CDC’s National Center for Immunizations and Respiratory Diseases funded the work. Several of the authors disclosed ties to industry, including GlaxoSmithKline, Abbvie, and Pfizer.

aotto@frontlinemedcom.com