From the seven studies included in this review for the case of MetHb and COVID-19, two were cross-sectional studies [19,21] and five case reports [17,18,20,22,23]. The case descriptions reported findings of one single patient [17,20,22,23] or three patients [18]. Five studies were published in journals [17,18,20,21,22] and two on the preprint server medRxiv (medrxiv.org) [19,23].