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American Journal of Epidemiology Advance Access originally published online on July 22, 2009
American Journal of Epidemiology 2009 170(5):650-656; doi:10.1093/aje/kwp173
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American Journal of Epidemiology © 2009 The Authors
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

PRACTICE OF EPIDEMIOLOGY

Influenza Vaccination and Mortality: Differentiating Vaccine Effects From Bias

Bruce Fireman, Janelle Lee, Ned Lewis, Oliver Bembom, Mark van der Laan and Roger Baxter

Correspondence to Bruce Fireman, Kaiser Permanente, 2000 Broadway, Oakland, CA 94612 (e-mail: bruce.fireman{at}kp.org).

Received for publication February 6, 2009. Accepted for publication June 2, 2009.

It is widely believed that influenza (flu) vaccination of the elderly reduces all-cause mortality, yet randomized trials for assessing vaccine effectiveness are not feasible and the observational research has been controversial. Efforts to differentiate vaccine effectiveness from selection bias have been problematic. The authors examined mortality before, during, and after 9 flu seasons in relation to time-varying vaccination status in an elderly California population in which 115,823 deaths occurred from 1996 to 2005, including 20,484 deaths during laboratory-defined flu seasons. Vaccine coverage averaged 63%; excess mortality when the flu virus was circulating averaged 7.8%. In analyses that omitted weeks when flu circulated, the odds ratio measuring the vaccination-mortality association increased monotonically from 0.34 early in November to 0.56 in January, 0.67 in April, and 0.76 in August. This reflects the trajectory of selection effects in the absence of flu. In analyses that included weeks with flu and adjustment for selection effects, flu season multiplied the odds ratio by 0.954. The corresponding vaccine effectiveness estimate was 4.6% (95% confidence interval: 0.7, 8.3). To differentiate vaccine effects from selection bias, the authors used logistic regression with a novel case-centered specification that may be useful in other population-based studies when the exposure-outcome association varies markedly over time.

aged; epidemiologic methods; influenza, human; influenza vaccines; mortality; selection bias

Abbreviations: VE, vaccine effectiveness


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