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Hospitalization and Skilled Nursing Care are Predictors of Influenza Vaccination Among Patients on Hemodialysis: Evidence of Confounding by Frailty

McGrath, Leah J. PhD*; Cole, Stephen R. PhD*; Kshirsagar, Abhijit V. MD; Weber, David J. MD*,‡; Stürmer, Til MD*; Brookhart, M. Alan PhD*

doi: 10.1097/MLR.0b013e3182a50297
Original Articles

Background: Observational studies of preventive medications, such as vaccinations, can suffer from the healthy-user bias because vaccinated patients may be healthier than unvaccinated patients. Indicators of health status and frailty suitable for attenuating this bias could be identified in administrative data.

Objective: To examine the association of baseline variables and time-dependent hospitalization and skilled nursing care with the receipt of influenza vaccination in patients with end-stage renal disease.

Research Design: Observational cohort study using United States Renal Data System files each year from 1999 to 2005.

Subjects: Population-based cohorts that included >115,000 adult, hemodialysis patients each year.

Measures: We estimated hazard ratios for the association of baseline variables and time-dependent hospitalization days and skilled nursing days with influenza vaccination, controlling for demographic and baseline health status variables.

Results: Vaccination coverage increased from 47% in 1999 to 60% in 2005. Patients with any length of hospitalization were less likely to be vaccinated, however, the association was stronger in patients with longer stays [15–25 d: hazard ratio=0.64 (95% confidence interval, 0.62–0.65); 26–30 d: 0.40 (0.38–0.42)]. Patients with any length of skilled nursing care of >1 day had similar estimates; these patients were also less likely to be vaccinated [26–30 d: 0.66 (0.64–0.69)].

Conclusions: Patients with long hospitalizations or skilled nursing stays were less likely to be vaccinated suggesting evidence of the healthy-user effect. These variables could be used to account for bias in studies of preventive services in patients on dialysis.

Supplemental Digital Content is available in the text.

Departments of *Epidemiology, Gillings School of Global Public Health

Medicine, Division of Nephrology and Hypertension

Medicine, Division of Infectious Diseases, University of North Carolina, Chapel Hill, NC

Supplemental Digital Content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal's Website,

Dr Weber serves as consultant and speaker for Merck, Pfizer, Sanofi Pastuer. Dr Sturmer received salary support from the UNC Center of Excellence in Pharmacoepidemiology and Public Health and receives salary support from unrestricted research grants from pharmaceutical companies to UNC. Dr Brookhart received research support from Amgen and has served as a scientific advisor for Amgen, Rockwell Medical, and Pfizer (honoraria declined, donated, or paid to institution). He received consulting fees from Crimson Health, DaVita Clinical Research, the Foundation for the National Institutes of Health, and World Health Information Consultants. All other authors declare no conflict of interest.

Reprints: Leah J. McGrath, PhD, Department of Epidemiology, Gillings School of Global Public Health, University of North Carolina, 725 Martin Luther King Jr., Blvd. CB#7590, Chapel Hill, NC 27599. E-mail:

© 2013 by Lippincott Williams & Wilkins.