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

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.