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Editor's Correspondence
July 23, 2007

To Rule Out Confounding, Observational Studies of Influenza Vaccine Need to Include Analyses During the “Preinfluenza Period”

Author Affiliations

Copyright 2007 American Medical Association. All Rights Reserved. Applicable FARS/DFARS Restrictions Apply to Government Use.2007

Arch Intern Med. 2007;167(14):1553-1554. doi:10.1001/archinte.167.14.1553-b

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.