Benefits of influenza vaccine in US elderly—appreciating issues of confounding bias and precision

5 Including the diagnosis code variables in analytic models, therefore, does not produce estimates of effect that are less biased; in fact, this method appears to result in estimates of effect that are, if anything, more biased than the unadjusted estimates. We agree that there are many published studies reporting a decreased risk of death and hospitalization in influenza vaccinated compared with unvaccinated seniors. Had we restricted our analyses to the influenza season, our findings could be similarly interpreted. However, reproducibility is not proof of validity. Our results clearly demonstrate that the differences in risk are not specific to the influenza season and so are biased. As we stated in our paper, we do not believe one can conclude that influenza immunization has no benefit with respect to mortality during the influenza season. Rather, we believe that because of the limitations of the data sources used, the non-randomized studies conducted to date do not provide a reliable basis for estimating the presence and/or size of this benefit. References