“…Thirteen studies were finally included, with a total of 241 COVID-19 patients belonging to the ozone as an adjuvant therapy group and 157 patients belonging to the standard therapy only as of the control group, respectively. In this review, the majority were observational studies (4 case-control studies, and 4 case-series) conducted in Spain [19] , [20] , [29] , Italy [18] , [28] , Turkey [21] , Egypt [27] , and China [30] . In addition, there is one clinical trial from India [22] , and four RCTs conducted in Italy [23] , [24] , India [25] and Turkey [26] .…”