In 2020, we suggested that umbilical cord-derived mesenchymal stem cell (UC-MSC) transplantation can significantly improve coronavirus disease 2019 (COVID-19) symptoms based on evidential relations (10.4252/wjsc.v12.i8.721). One year later, this review aims to summarize and update the clinical evidence regarding UC-MSC usage in COVID-19 treatment. The publications on applications of UC-MSCs were searched in the PubMed, Web of Science, and Google scholar databases with the keywords"umbilical

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... -derived mesenchymal stem cells" and"COVID-19" . All publications about clinical studies, from case reports to randomized controlled trials (RCTs), were used as clinical evidence in this review. The results showed 16 publications (4 randomized clinical trials, 3 pilot studies/phase 1 clinical trials, 3 case series reports, and 6 case reports) with a total of 395 COVID-19 patients that were provided with UC-MSC transplantation. All publications demonstrated that UC-MSC transplantation is safe, well tolerated, improved COVID-19 symptoms, and significantly decreased mortality. These findings support our suggestion for the usage of off-the-shelf UC-MSCs for COVID-19 as an adjuvant therapy.