Reversion of a vaccine organism to its original pathogenic form is known to have happened with which vaccine?

1. A live vaccine virus could in theory revert to the pathogenic form it was weakened from. 2. In practice this is known to have happened only with live oral polio vaccine. 3. That vaccine is no longer available in the United States. 4. Measles, mumps and rubella, rotavirus and intranasal influenza are also live vaccines, which makes them tempting answers. 5. None of those three is named as having reverted, so choosing them imports a risk that has not been observed. 6. The separate hazard of uncontrolled replication, which can be severe or fatal, is confined to people with a weakened immune system. _Source: CDC Epidemiology and Prevention of Vaccine-Preventable Diseases (Pink Book), section Live, Attenuated Vaccines_