As a footnote, I came across a couple interesting papers on the topic of vaccine contamination from around 2012. They talk about the usual development time being 8-10 years, and about a few recent cases of vaccine contamination.

Extraneous agent detection in vaccines: https://www.researchgate.net/profile/Attila_Farsang_Dr/publi...

Detection of Avian Retroviruses in Vaccines: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3647912/