Evidence of bias in studies of influenza vaccine effectiveness in elderly patients

Although studies have shown influenza vaccines to be effec-tive in preventing death in the elderly population, these find-ings may be the result of selection bias. We examined the relationship between vaccination, age, underlying morbidity, and probability of death in the upcoming year. Vaccination coverage varied in a curvilinear fashion with age, morbidity, and risk of death. Forgoing vaccination predicted death in those who had received vaccinations in the previous 5 years, but it predicted survival in patients who had never before received a vaccination. We conclude that bias is inherent in studies of influenza vaccination and death among elderly patients. Influenza is associated with significant morbidity [1, 2]. Vac-cines have been available for many years, and for over 4 decades, they have been recommended in the United States for indi