1918 Pandemic influenza was most probably swine influenza spread by the Rockefeller Institute for Medical Research and its experimental bacterial meningococcal vaccine.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2118275/

https://academic.oup.com/aje/article/187/12/2498/5059949

https://www.historyofvaccines.org/content/antitoxin-production-united-states

https://www.lewrockwell.com/2020/03/no_author/did-a-vaccine-experiment-on-u-s-soldiers-cause-the-spanish-flu/