Paranjpe et al. studied 2773 individuals hospitalized within the Mount Sinai Health System, New York City [3]. Overall, 786 individuals received treatment-dose AC during their hospital course and median time from admission to AC initiation was 2 days (interquartile range 0–5 days). Study participants were followed from hospital admission (T0) until discharge, death, or end of study. The authors compared mortality among AC users versus non-users and found similar mortality (22.5% versus 22.8%). In a sub-analysis among individuals receiving mechanical ventilation, AC was associated with greater benefit (mortality 29.1% versus 67.2%). However, as T0 was date of admission and AC initiation was delayed, the authors introduced immortal person-time among AC users, thereby conferring an artificial survival advantage to the AC group. Immortal time bias (or survivor treatment selection bias) can occur in survival analyses where patients who live longer are more likely to receive treatment than patients who suffer an early death [4]. As an example, Kaplan–Meier survival curves in the paper by Paranjpe et al. give the false illusion of improved survival among AC users when in fact ~25% of AC users were not at risk of death until after day 5 and all non-users were at risk from day 0.