> Genetic analysis and several unusual characteristics of the 1977 Russian flu have prompted many researchers to speculate that the virus was released to the public through a laboratory accident or resulted from a live-vaccine trial escape.

Speculate. Covid shows how easy it is to do just that even if you have current knowledge.

It's not impossible that origin is other than zoonotic however it's very unlikely because that exactly how (nearly?) all viruses came to us.