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While several individual studies have compared outcomes of transferred and non-transferred patients, no systematic synthesis of this evidence exists. This systematic review aims to determine whether inter-hospital ICU-to-ICU transfer of adult COVID-19 patients is associated with increased mortality or other adverse clinical outcomes. We systematically searched PubMed, Web of Science, and Scopus for observational studies comparing clinical outcomes of adult COVID-19 patients who underwent inter-hospital ICU-to-ICU transfer with non-transferred ICU patients. Two reviewers screened, selected, and extracted data independently and in duplicate. Risk of bias was assessed using the Newcastle-Ottawa Scale and ROBINS-I, and certainty of evidence using GRADE. Exploratory random-effects meta-analyses were performed separately for each effect measure, with Hartung-Knapp and crude-effect sensitivity analyses. Nine observational studies from seven countries were included (approximately 6,100 transferred and 29,200 non-transferred ICU patients). None of the adjusted effect estimates showed a statistically significant mortality disadvantage for transferred patients. Pooled adjusted subgroup estimates were an odds ratio of 1.29 (95% CI 0.84 to 1.98; k\u2009=\u20093) and a hazard ratio of 0.71 (95% CI 0.34 to 1.46; k\u2009=\u20092). Length of stay was longer in transferred patients in most studies. The available observational evidence did not show a clear increase in mortality among transferred compared with non-transferred COVID-19 ICU patients. These findings support the use of inter-hospital transfer as a safe strategy for managing ICU surge capacity. 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