This 1920 medical study is likely the source for Trump discussing injecting ‘disinfectants’ to treat Coronavirus. It seems there were promising treatments for influenza (and influenzal pneumonia) early in the century that involved both injecting hydrogen peroxide intravenously and using UV light to eradicate the virus. Both were extremely small groups that were tested, showed convincing results, but were not brought into large trials because antibiotics were introduced into widespread use just a few short years later.

Here is the medical paper for influenza/ pneumonia and treatment with hydrogen peroxide, a common disinfectant:

https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(01)11118-9/fulltext

Modern medical journals have cited these studies as approaches that hold promise to treat Coronavirus (before COVID-19!)

https://clinmedjournals.org/articles/jide/journal-of-infectious-diseases-and-epidemiology-jide-6-113.php?jid=jide