Although pregnancy, obesity and bacterial co-infection have been associated with poor prognosis during the pandemic (H1N1) 2009 [5,21–23], we did not find significant differences in the prevalence of these factors between the two study periods. Moreover, we performed a subgroup analysis of patients with complicated pneumonia to control for reasons other than disease severity that might have contributed to site-of-care decisions. Differences found between periods in this subgroup of patients with severe pneumonia were similar to those encountered in the entire population. On the other hand, it has recently been reported that the pandemic influenza A (H1N1) 2009 virus has remained genetically stable, with no increase in virulence since its origin [24,25]. Consequently, other factors may have been involved in the striking changes documented in the present study