Seasonal variation in community-acquired pneumonia

Purpose: To determine and describe the seasonal frequency of all case of community-acquired pneumonia (CAP) and of its specific etiologic agents. Methods: All 346 adult patients who were admitted to a regional hospital in southern Israel for CAP over a period of one year were included in a prospective study. As part of the study a comprehensive diagnostic work-up was performed to identify the specific causative agents for CAP. The study findings were analyzed and are presented in terms of the seasonal distribution of the disease. Results: Thirty-four percent of CAP cases occurred in the spring and only 18% in the fall (p=0.036). Respiratory viruses CAP occurred mainly in the winter and spring (p=0.009) wnile Mycoplasma pneumoniae CAP reached its peak incidence of 41% in the spring compared with 15% during the winter months (p=0.054). No significant seasonal predominance was observed for the other specific aetiological agents of CAP. In 58% of the CAP patients who were hospitalized in the fall we identified a pneumococcal aetiology while this agent was found in only 32% of the cases in the winter. Conclusions: In contrast with popular public opinion, winter with its low temperatures is not the main reason for the development of community-acquired pneumonia. Most of the specific aetiologies for CAP, with the exception of respiratory viruses and M. pneumoniae, have no seasonal pre-dilection. Clinical implications: The results of our study can aid in the differential diagnosis of the etiological agents of CAP and the initial therapeutic decisions.