"Jonas Salk attributed this finding to changes in the virus that had occurred since the previous year."
Historical Perspective — Emergence of Influenza A (H1N1) VirusesZimmer 2009,
http://www.nejm.org/doi/full/10.1056/NEJMra0904322
The total influenza vaccine failure of 1947 revisited: Major intrasubtypic antigenic change can explain failure of vaccine in a post-World War II epidemicKilbourne 2001,
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC125033/
