The 1977 Russian Flu pandemic was genetically near-identical to a strain from 1950, without the expected mutations that should have appeared after 27 years of undetected circulation among humans or animals. It's been widely suspected in mainstream literature to have escaped from a research or vaccine manufacturing accident, to the point that the NEJM casually wrote:
> The reemergence was probably an accidental release from a laboratory source in the setting of waning population immunity to H1 and N1 antigens
https://www.nejm.org/doi/full/10.1056/NEJMra0904322
But at the time, the WHO said:
> Laboratory contamination can be excluded because the laboratories concerned either had never kept H1N1 virus or had not worked with it for a long time.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2395678/pdf/bul...
And it's absolutely bizarre to me that people are asking why the origin even matters. After thousands of people died in Bhopal, did it matter whether better chemical safety standards could have prevented that? So with millions dead now, how could you possibly not wonder whether our current standards for the sampling and manipulation of poorly-understood pandemic-candidate viruses are adequate?
https://project-evidence.github.io/
Summary (IIRC): The most likely explanation of its origins is a person who collected bats for a lab in Wuhan contracted the disease in the cave where bats were collected.