A hospital in Hubei, China that was treating covid-1i patients gave ~2,400 medical staff interferon by nasal drops. The treated group had zero COVID-19 cases. It wasn't a blind study, and they didn't have a real control group. But a supposedly comparable group of medical staff treating covid-19 patients had a ~50% infection rate.
https://www.medrxiv.org/content/10.1101/...20061473v1
https://www.medrxiv.org/content/10.1101/...20061473v1