Back in the 1950s, Sabin and Salk developed polio vaccines. Salk’s vaccine was inactivated. Sabin’s vaccine was live, but used a weakened strain, strong enough to cause an immune reaction, but weak enough not to cause polio. The live version was also infectious, which amplified its protective effect in the community.
The virus was weakened by passage through a number of cell cultures (live monkey, monkey testicular cultures, monkey kidney cells, etc).
Being a virus, it was grown in cell culture, derived from rhesus monkeys. The problem with all those live cell cultures was the possibility of picking up some other monkey virus. And that happened: 10-30 million Americans received vaccine contaminated with SV40 (Simian vacuolating virus 40) between 1955 and 1963. It is suspected that versions of the vaccine produced produced in the East Bloc may have been contaminated far longer, as late as 1980, which could have exposed several hundred million more people to SV40.
SV40 is pretty good at causing cancer in hamsters: there it causes sarcomas.
Does it cause cancer in humans? Some people claim to have found it in some cases of osteosarcoma, non-Hodgkins lymphoma, and mesothelioma (similar to the kinds of cancer in infected hamsters).
Currently it’s not clear whether SV40 causes cancer in humans – but it is clear that at worst, it causes very few such cancers.
It could have been worse. What if the contaminant had been HIV, instead of SV-40?
It wouldn’t have killed everybody. A fair fraction of people contracting HIV have flu-like symptoms a few weeks after. Usually the serious immune deficiency does not develop for at least three years – if the contaminant had been HIV, presumably the powers that be would have realized that something had gone wrong within a couple of years after the beginning of mass administration, because of the relatively few but dramatic early immunodeficiency cases. Even allowing for frantic defensive nonsense emitted by those people that stood to look bad from such a disaster. It might have been detected in the Francis Field Trial, in 1954 and 1955, where about 440,000 people were injected. I doubt if it would have killed more than a million people – mostly kids. No more than 10, 20 million killed, tops. I’m not counting later cases resulting from transmission via needle-sharing and sodomy.
The risk of picking up an unknown virus from cell culture had been discussed. Salk and Sabin were not as careful as they should have been. Overeager. Ambitious. Polio was scary and well worth fighting, but the potential downside went way, way down.
This does show that the price of insufficient medical vigilance can be very high.
Probably the largest existing example of death-by-medical-oopsie is the very high incidence of Hepatitis C in Egypt, which was spread by a mass anti-schistosomiasis campaign between 1960 and 1980, in which a goodly fraction of everyone in Egypt (~15%) seem to have been inoculated with the same needle, setting them up for cirrhosis and liver cancer.