Most sources discount it, but there seems to be significant evidence for a possible link:

One theory of the origin of AIDS is that it developed from contaminated vaccines used in the world's first mass immunisation for polio. There are a number of reasons why this theory is plausible enough to be worthy of further investigation.

The location coincides dramatically. The earliest known cases of AIDS occurred in central Africa, in the same regions where Koprowski's polio vaccine was given to over a million people in 1957-1960.

The timing coincides. There is no documented case of HIV infection or AIDS before 1959. Centuries of the slave trade and European exploitation of Africa exposed Africans and others to all other diseases then known; it is implausible that HIV could have been present and spreading in Africa without being recognised.

Polio vaccines are grown (cultured) on monkey kidneys which could have been contaminated by SIVs. Polio vaccines could not be screened for SIV contamination before 1985.

Another monkey virus, SV-40, is known to have been passed to humans through polio vaccines. A specific pool of Koprowski's vaccine was later shown to have been contaminated by an unknown virus.

In order for a virus to infect a different species, it is helpful to reduce the resistance of the new host's immune system. Koprowski's polio vaccine was given to many children less than one month old, before their immune systems were fully developed. Indeed, in one trial, infants were given 15 times the standard dose in order to ensure effective immunisation.

If this theory is correct, it has serious ethical, health and policy implications. In particular, it points to the danger of interspecies transfer of material through vaccinations, organ transplants, etc., which could lead to new variants of AIDS as well as other new diseases. As well, studying the theory may lead to insights about responding to AIDS and preventing new diseases.

However, there has been no sustained attempt to test the theory. This could be done, for example, by testing stocks of polio vaccine for the presence of SIV. An offer to undertake tests was made as early as 1991; only in 2000 were some samples tested, and then only US-made vaccine. Another possibility would be to test stored blood samples in Africa from before 1950. If HIV is found, this would undermine the theory.