As reported in Science (Division of Biologics Standards:  The Boat That Never Rocked, vol. 175, pp.1225-1230, 1972):

“There can be few graver opportunities for man-made disaster than the mass immunization campaigns that are now routine in many countries.  Should the vaccine preparations become contaminated with an undetected agent present in the host cells, such as a cancer-causing virus, a whole generation of vaccines could be put in jeopardy.  This, of course, is no science fiction writer’s horror story – it has already happened once; millions of people have been injected with a monkey virus known as SV40, which was found in 1961 to be contaminating polio and adenovirus vaccines.  The virus causes cancer in hamsters; no one yet knows what it many do in man…….The importance attached to presenting an unruffled surface to the public is exemplified by the SV40 incident of 1961; even when the contaminating virus was found to be oncogenic in hamsters, the DBS and its expert advisory committee decided to leave existing stocks on the market rather than risk eroding public confidence by a recall.”