Nasal and throat swab specimens were collected from all highrisk and intermediate-risk patients. Swabs were combined in phosphate-buffered saline viral transport media and split into aliquots for influenza testing. One aliquot was tested by rapid diagnostic test (QuickVue Influenza Test, Quidel, San Diego, CA) and immunofluorescence at the hospital. A second aliquot was sent frozen at 270uC to the National Public Health (InDRE) laboratory in Mexico City. InDRE tested the samples with realtime RT-PCR (rRT-PCR) using a multiplex assay and 4 sets of primers (i.e. influenza A, universal swine, 2009 pandemic influenza A (H1N1), and a control for human genetic material) [19] . Each hospitalized patient had a chest x-ray and a chest CT scan performed at admission.