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

Bjarnason, Agnar; Westin, Johan; Lindh, Magnus; Andersson, Lars-Magnus; Kristinsson, Karl G; Löve, Arthur; Baldursson, Olafur; Gottfredsson, Magnus

Uniquely, the etiology of pneumonia was examined in the population of Reykjavik in 2 simultaneous studies undertaken 25 years before the current work, which found that S pneumoniae caused 39% and 26% of cases [25, 26] . This apparent decrease may in part be due to the fact that smoking, a risk factor for both CAP and invasive pneumococcal disease [27, 28] , decreased by 50% in the population in the intervening period [29] . The decrease in S pneumoniae between these studies and the current one is also in line with the decline in rates of pneumococcal pneumonia during the last century, as reviewed succinctly by Musher et al [30] . This has not been previously illustrated by repeated studies of the same population, to the author's knowledge. The rate of M pneumoniae was relatively high in this study, i.e., identified in 36 cases (12%). In comparison, Jain et al [12] and Gadsby et al [13] identified M pneumoniae in 2% of their respective cohorts, whereas Holter et al [11] found this pathogen in 4% of patients. This result may be due to an ongoing local epidemic during the study period, but this question requires further study. Younger patients were relatively more likely to have M pneumoniae or influenza infections in this study. Although M pneumoniae infection is known to be common in younger individuals, approximately one third of cases were among older patients in this study, and the highest incidence was in this group [3] . Mycoplasma pneumoniae is also implicated in severe pneumonia; Miyashita et al [31] found that 6% of patients had severe disease requiring ICU admission. In this study, 4 (11%) of patients with M pneumoniae received ICU care. In comparison, 5 patients (8%) with S pneumoniae were admitted to ICUs.