BACKGROUND: In a context of overwhelming demand, mass transfers between intensive care units (ICU) were organized in France during the first COVID-19 epidemic wave (spring 2020). According to early reports, transferred patients experienced a 3-to-4-fold lower ICU case fatality. It is not known whether this difference stems only from the selection of healthier patients for transfer. RESEARCH QUESTION: Is the 28-day ICU case fatality of transferred patients different from that of matched control (not transferred) patients? STUDY DESIGN AND METHODS: Multicenter retrospective cohort study that included 285 transferred patients, and 667 control (not transferred) patients admitted simultaneously (± 2 days) to the same "origin" ICU and alive five days after the transfer date. The 28-day ICU case fatality and clinical events during ICU stay were compared in transferred and control patients. RESULTS: At ICU admission, age, COVID-19 severity, comorbidities and Simplified Acute Physiology Score II (SAPS II) were similar, but transferred patients were lighter (81 versus 89 kg, p<0.0001) and more autonomous than the matched control patients (64.5 versus 55.0%, p=0.01). Case fatality was approximately 7-fold lower in transferred patients (adjusted incidence rate ratio = 0.14 [95% CI: 0.10-0.19]). ICU stay was longer and delirium, psychiatric disorders and neuromuscular blockade exposure were more frequent in transferred patients than controls. Conversely, acute kidney injury was more frequent in controls (51.5 versus 37.7%, p<0.0001). INTERPRETATION: Although the selection of healthier patients likely contributed to better survival, removal from an overcrowded care environment probably also explains the large survival benefit associated with transfer. By reducing workload, mass transfers might have also benefited patients who remained in origin ICUs. Organizing mass transfers as early as possible may be an appropriate strategy for mitigating the impact of an overwhelming intensive care demand.