Do specific virus-bacteria pairings drive clinical outcomes of pneumonia?

Bacterial pneumonia is a common contributor to severe outcomes of influenza. Epidemiological data suggest that the incidence, severity and associated bacterial pathogens differ between epidemics and by geographical location within epidemics. Data from animal models demonstrate that differences in both viral and bacterial strains alter the incidence and outcomes of pneumonia. For influenza viruses, evolutionary changes to specific virulence factors appear to alter the ability of viruses within particular lineages to prime the host for secondary bacterial infection. Although bacterial strains differ considerably in disease potential in the setting of viral co-infection, the bacterial virulence factors underlying this finding are currently unknown. The hypothesis that geographical variation exists in the prevalence of bacterial strains expressing factors that enable efficient disease potentiation during viral epidemics should be considered as one explanation for regional differences in severity. This would have implications for surveillance, vaccine development, and the conduct of clinical trials for the prevention or treatment of pneumonia.