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Given the stark differences between each arm of the study, "I think it is difficult to compare the mild disease portion of the study to the severe disease cohort," said Dr. Maria Nagel, a professor of neurology and ophthalmology at the University of Colorado School of Medicine, who was not involved in the study. In other words, brain changes seen in mild infection may not be driven by the same mechanisms as those seen in tissue from people who died of COVID-19, she told Live Science in an email.