FWIW, friend-of-a-friend who had covid took 100mg of fluvoxamine together with melatonin and a range of cold & flu medication, and got something that was probably serotonin syndrome. Also strong drowsiness (slept for 10h) which is normally anti-correlated with serotonin syndrome, but which was plausibly caused by the melatonin+fluvoxamine interaction.
Serotonin syndrome is pretty bad. It’s unclear what caused it in this case. (It’s normally super rare if the only seratonin-boosting thing you’re taking is a single SSRI.) But we know that fluvoxamine+melatonin interacts to increase melatonin levels a ton. And I think melatonin-levels have non-0 interaction with serotonin-levels. So personally, I would not simultaneously take melatonin and fluvoxamine as a response to covid.