ABSTRACT Background Nudges have been proposed as an effective tool to promote influenza vaccination of healthcare workers. To be successful, nudges must match the needs of the target healthcare workers population and be acceptable. Objective To evaluate the effectiveness and the acceptability of an opt-out nudge promoting influenza vaccination among medical residents. Methods The hypothesis were that an opt-out nudge would be effective, better accepted when applied to patients than to residents, and that prior exposure to a nudge and being vaccinated increase its acceptability and residents’ sense of autonomy (the feeling of being in control of their choice about whether to get vaccinated). Residents were randomly divided into two parallel experimental arms: a nudge group and a control group. The nudge consisted in offering participants an appointment for a flu shot, while leaving them the choice to refuse or to reschedule it. Results The analysis included 260 residents. Residents in nudge group were more likely to be vaccinated than residents in control group. There was a strong consensus among the residents that it is very acceptable to nudge their peers and patients. Acceptability for residents and patients did not differ. Acceptability was better among residents exposed to the nudge and residents who were vaccinated. Residents considered that the nudge does not reduce their control over whether to get a flu shot. The sense of autonomy was associated with nudge’s acceptability. Conclusion An opt-out nudge to promote influenza vaccination among medical residents can be effective and very well accepted. These data suggest that this approach can complement other vaccination promoting interventions and be eventually extended to other healthcare workers’ categories and to general population, but should consider its ethical implications. More studies are needed to assess the nudge’s effectiveness and acceptability on other populations. Key messages Nudging is one of the most efficient techniques to improve vaccination coverage. Changing defaults may be effective in promoting vaccination and well accepted. Changing defaults does not lower the feeling of control over choices. The sense of autonomy related to a nudge is correlated to its acceptability. Behavior adoption increases the sense of autonomy related to a nudge. Using defaults for nudge should take into account the ethical implications.