The new theory behind some of the symptoms of Long Covid is that the virus stays in the gut. This article from the New York Times explains that the residual virus tends up depleting seratonin levels.
The following quote is important. Peripheral serotonin deficiency impairs cognition via reduced vagal signaling
I went to a conference on Neuromodulation about 18 months ago and there was a presentation about using Vagal stimulation in the hospital on Covid Patients. Now there seems to be a good rationale for doing this, as well as for treating with acupuncture to try to improve vagal signalling.
Wong, A. C., Devason, A. S., Umana, I. C., Cox, T. O., Dohnalová, L., Litichevskiy, L., Perla, J., Lundgren, P., Etwebi, Z., Izzo, L. T., Kim, J., Tetlak, M., Descamps, H. C., Park, S. L., Wisser, S., McKnight, A. D., Pardy, R. D., Kim, J., Blank, N., . . . Levy, M. (2023). Serotonin reduction in post-acute sequelae of viral infection. Cell. https://doi.org/10.1016/j.cell.2023.09.013