Between 1955-1963, polio vaccines produced using rhesus monkey kidney cells were contaminated with Simian Virus 40 (SV40). The CDC acknowledged that 10-30 million Americans may have received SV40-contaminated vaccine (some estimates go to 98 million). SV40 has been found in certain human cancers including mesothelioma and brain tumors. However, epidemiological studies have produced mixed results on whether the contaminated vaccines increased cancer rates in recipients.