A federal government agency knowingly released polio vaccine contaminated with a monkey virus in the 1960s that has since been linked to a range of cancers, including mesothelioma. The virus contaminated at least four batches of vaccine totalling almost three million doses between 1956 and 1962. Two of the batches were released after testing positive to contamination. The other two were released before tests could be done. An unknown number of earlier batches were also almost certainly contaminated. It is estimated that thirty million people worldwide were recipients of the contaminated vaccine.