If you or someone you know is experiencing “brain fog” after COVID-19, scientists now have a possible explanation — and it might not bring much comfort.

Researchers in Germany found that part of the virus, the spike protein, remains in the brain long after the virus clears out.

These investigators discovered the spike protein from the virus in brain tissue of animals and people after death. The finding suggests these virus fragments build up, stick around, and trigger inflammation that causes long COVID symptoms.

About 15% of COVID patients continue to have long-term effects of the infection despite their recovery, said senior study author Ali Ertürk, PhD, director of the Institute for Tissue Engineering and Regenerative Medicine at the Helmholtz Center Munich in Germany.