Upon the discovery that SV40 was an animal carcinogen that had found its way into the polio vaccines, a new federal law was passed in 1961 that required that no vaccines contain this virus. However, this law did not require that SV40 contaminated vaccines be thrown away or that the contaminated seed material (used to make all polio vaccines for the next four decades) be discarded. As a result, known SV40 contaminated vaccines were injected into children up until 1963. In addition, it has been alleged that there have been SV40-contaminated batches of oral polio vaccine administered to some children until the end of the 1990's.