We retrieved data on a cohort of medical patients at a regional Israeli hospital. The dependent variable was non-COVID-19 hospital mortality; the independent variables were vaccination status, age, and laboratory data. Serum sodium, age, serum creatinine, and COVID-19 vaccination status were the main independent variables associated with non-COVID-19 mortality. The odds ratio for in-hospital deaths of non-vaccinated patients was 2.01 (1.65–2.44) (unadjusted) and 1.61 (1.29–2.03) after adjustment for the independent variables. This “healthy adherer effect” may confound observational assessments of the clinical efficacy of COVID-19 vaccines.