Nearly 50% of people with long COVID experience brain fog, making it a common symptom of the condition. But the exact cause of long COVID brain fog has been a mystery. Now, a small study suggests this brain fog could be due to leaky blood vessels in the brain.

The small study, which was published in the journal Nature Neuroscience, analyzed blood samples—specifically, serum and plasma—from 76 patients who were hospitalized with COVID-19 in March or April 2020, along with those from 25 people taken before the pandemic. The researchers discovered that people who said they had brain fog had higher levels of a protein in their blood called S100β than people who didn’t have brain fog.

S100β is made by cells in the brain and isn’t normally found in the blood. That suggests that the patients had a breakdown in the blood-brain barrier, which blocks certain substances from getting to the brain and spinal cord, the researchers noted. To read the full story.