> ”Pretty much every modern influenza variant is a derivative of the Spanish flu”

Not really. While it was true for much of the last century that many “seasonal flu” viruses were direct descendents from H1N1 Spanish flu, the H1N1 that caused the 2009 “swine flu” pandemic was a distinct strain combining new genes of pig, avian, and human flu origins. Since then, derivatives of that virus have replaced earlier ones as the most common circulating H1N1 types.

And right now the most common circulating influenza is actually H3N2 from a completely different (avian) lineage with origins in the 1968 pandemic.

I'd further note that the H1N1 flu is widely believed to have gone extinct in the wild, then been reintroduced in 1977 from a laboratory source, probably through a failed vaccine trial:

> The reemergence was probably an accidental release from a laboratory source in the setting of waning population immunity to H1 and N1 antigens.19,31

https://www.nejm.org/doi/full/10.1056/NEJMra0904322

This explanation is now printed uncontroversially in major journals, though at the time the WHO rejected it:

> 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://pmc.ncbi.nlm.nih.gov/articles/PMC2395678/pdf/bullwho...