In a study comparing 46 severe COVID-19 patients with 460 matched controls, British researchers found the mental impact of severe COVID-19 six months later can be the equivalent to aging 20 years – going from 50 to 70 years old – or losing 10 IQ points. Previous research has shown during severe COVID, the brain decreases glucose consumption in the frontoparietal network, which is involved in attention, problem solving and working memory. 

It's also known the virus can directly affect the brain. But the research suggests the likely culprit isn't direct infection, but a combination of factors: including reduced oxygen or blood supply to the brain; clotting of vessels; and microscopic bleeds. The full report has been published in Clinical Medicine.