Mortality reduction with influenza vaccine in patients with pneumonia outside “flu” season: pleiotropic benefits or residual confounding?

Objectives: To evaluate unrecognized confounding by hypothesizing the presence of a 50% mortality reduction with vaccination for patients with pneumonia outside of influenza season…. Conclusions: The 51% reduction in mortality with vaccination initially observed in patients with pneumonia who did not have influenza was most likely a result of confounding. Previous observational studies may have overestimated mortality benefits of influenza vaccination.

Abstract. American journal of respiratory and critical care medicine 2008; 178(5):527-33