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.
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.
Observational cohort study using United States Renal Data System files each year from 1999 to 2005.
Population-based cohorts that included >115,000 adult, hemodialysis patients each year.
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.
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)].
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.