Rats. The gist: residual spike protein has been found in the gut and they believe that may cause the over-production of interferon causing inflammation that contributes to cells not being able to absorb and circulate serotonin. May not help all types of long Covid but may help the brain fog, energy crashes, and fatigue type. More research going on to see if treatment with serotonin or tryptophan helps alleviate symptoms. Tryptophan does interact with other meds and conditions so do read up before trying it and talk to your doctor if you can try for short period (never more than 3 weeks). This is first integrated explanation of all that I have — and there are tests (low serotonin, virus in stools, and/or high interferon levels). God bless.