Clinical factors predictive of pneumonia caused by pandemic 2009 H1N1 influenza virus.

Pneumonia was the most common cause of death during the 2009 pandemic H1N1 influenza virus infection. Clinical risk factors for pneumonia caused by this virus are limited. We enrolled consecutive patients treated at the H1N1 Clinic in Thungsong Hospital in Nakhon Si Thammarat, Thailand, during June-December 2009 who had positive polymerase chain reaction results for H1N1 virus. Clinical features for patients given a diagnosis with and without pneumonia were studied. There were 441 patients with positive polymerase chain reaction results for H1N1 virus. Of these patients, 51 (11.56%) had pneumonia. Three independent clinical factors for H1N1 pneumonia were myalgia, dyspnea, and an absolute neutrophil count > 7,700 cells/μL. Adjusted odds ratios (95% confidence intervals) for these three variables were 0.413 (0.173-0.988), 2.625 (1.230-5.604), and 4.475 (1.882-10.644), respectively. Clinical features may be a useful tool for predicting risk for pneumonia caused by H1N1 virus.