Seasonal variation in community-acquired pneumonia.

Seasonal variation in the incidence of disease in general, and of infectious diseases in particular, is vitally important information for epidemiologists and for clinicians who deal with these diseases. In relation to community-acquired pneumonia (CAP), the seasonal distribution of the disease and, in particular, of each of the specific aetiological agents is important. All 346 adult patients who were admitted to a regional hospital in southern Israel for CAP over a period of 1 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 analysed and are presented in terms of the seasonal distribution of the disease. Thirty four percent of CAP cases occurred in the spring and only 18% in the autumn (p = 0.036). Respiratory viruses CAP were predominant in the winter and spring (p = 0.009), whilst 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 autumn we identified a pneumococcal aetiology, whilst this agent was found in only 32% of the cases in the winter. 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 community-acquired pneumonia, with the exception of respiratory viruses and Mycoplasma pneumoniae, have no seasonal predilection.