Incidence, Etiology, and Outcomes of Community-Acquired Pneumonia: A Population-Based Study

Background. The microbial etiology of community-acquired pneumonia (CAP) is often unclear in clinical practice, and previous studies have produced variable results. Population-based studies examining etiology and incidence are lacking. This study examined the incidence and etiology of CAP requiring hospitalization in a population-based cohort as well as risk factors and outcomes for specific etiologies. Methods. Consecutive admissions due to CAP in Reykjavik, Iceland were studied. Etiologic testing was performed with cultures, urine-antigen detection, and polymerase chain reaction analysis of airway samples. Outcomes were length of stay, intensive care unit admission, assisted ventilation, and mortality. Results. The inclusion rate was 95%. The incidence of CAP requiring hospitalization was 20.6 cases per 10 000 adults/year. A potential pathogen was detected in 52% (164 of 310) of admissions and in 74% (43 of 58) with complete sample sets. Streptococcus pneumoniae was the most common pathogen (61 of 310, 20%; incidence: 4.1/10 000). Viruses were identified in 15% (47 of 310; incidence: 3.1/10 000), Mycoplasma pneumoniae were identified in 12% (36 of 310; incidence: 2.4/10 000), and multiple pathogens were identified in 10% (30 of 310; incidence: 2.0/10 000). Recent antimicrobial therapy was associated with increased detection of M pneumoniae (P<.001), whereas a lack of recent antimicrobial therapy was associated with increased detection of S pneumoniae (P=.02). Symptoms and outcomes were similar irrespective of microbial etiology. Conclusions. Pneumococci, M pneumoniae, and viruses are the most common pathogens associated with CAP requiring hospital admission, and they all have a similar incidence that increases with age. Symptoms do not correlate with specific agents, and outcomes are similar irrespective of pathogens identified.