[goda90]

It's likely that sometimes it goes poorly in bigger sample sizes. Maybe the viruses have a chance of causing a cytokine storm which can kill the patient.

[gpm]

There's a long history of OVT clinical trials, if someone's bored and interested they could try tracking down why they (mostly*) failed.

See table 1 for a list (as of 2021, probably incomplete) https://pmc.ncbi.nlm.nih.gov/articles/PMC7913179/

* As the article points out, there is one approved OVT therapy.