Safe? Don't think so.
The fact that the polio vaccine in the 1950's was contaminated with cancer causing viruses was mentioned on the CDC's website......... only to be deleted (=covered up) in 2013, whereafter the information can only be found on the Internet Archive:

https://web.archive.org/web/20110307094146/http://www.cdc.gov/vaccinesafety/updates/archive/polio_and_cancer_factsheet.htm