"Back mutations occur when the vaccine undergoes mutations in the host such that it readapts to the host and can again cause disease, which can then be spread to other humans in an epidemic. This happened as recently as 2007 in Nigeria where mutations in a polio vaccine led to an epidemic of polio in that country."
CONCEPTS OF BIOLOGY-1ST CANADIAN EDITION https://opentextbc.ca/biology/chapter/12-1-viruses/