Serotonin reduction may be triggered by remnants of the virus lingering in the gut. Depleted serotonin could explain memory problems and some neurological and cognitive symptoms of long Covid.

“The research could point the way toward possible treatments, including medications that boost serotonin. And the authors said the biological pathway that their research outlines could unite many of the major theories of what causes long Covid: lingering remnants of the virus, inflammation, increased blood clotting and dysfunction of the autonomic nervous system.”

https://www.cell.com/cell/fulltext/S0092-8674(23)01034-6