In their study of adults (primarily older adults) hospitalized for pneumonia during influenza season, Spaude and colleagues1 found that prior receipt of influenza vaccine was associated with a 39% reduction in risk of in-hospital mortality. The authors interpreted this association as evidence of a protective effect of vaccine. An alternate explanation is that persons at lower risk of death were more likely to have been vaccinated—in other words, that the observed association was due to confounding.
Jackson ML, Weiss NS, Nelson JC, Jackson LA. To Rule Out Confounding, Observational Studies of Influenza Vaccine Need to Include Analyses During the “Preinfluenza Period”. Arch Intern Med. 2007;167(14):1553–1554. doi:10.1001/archinte.167.14.1553-b