Middle East respiratory syndrome coronavirus continues to circulate in the Middle East. During a recent outbreak in Korea, changes in MERS coronavirus viral load were determined during the course of illness in 17 patients. To the Editor: The outbreak of Middle East respiratory syndrome coronavirus (MERS-CoV) infection in South Korea involved 186 patients and resulted in 38 deaths, with four large hospital outbreaks accounting for 82% of the total cases. 1 , 2 Here, we report changes in viral load over time in patients with MERS. We included all patients who were admitted to three Seoul National University–affiliated hospitals; the institutional review boards of these hospitals approved this study and waived the need for written informed consent on public health grounds. The patients were categorized into a group with severe disease (severe group) or a group . . .