Unique strains of SV40 in commercial poliovaccines from 1955 not readily identifiable with current testing for SV40 infection

SV40 was first identified as a contaminant of poliovaccines used from 1955 until 1963. Recently, SV40 has been detected in several human tumors. The v irus detected in human tumors often contained only one 72-bp enhancer in th e regulatory region, in contrast to the SV40 originally isolated from polio vaccines, which contained two 72-bp enhancers. The origin of viruses with o ne 72-bp enhancer in humans was unknown, because it was thought that these viruses were not present in poliovaccines. It was also thought that all pol iovaccine vials produced from 1955 until 1963 had been discarded, thus the possibility that one 72-bp virions contaminated those vials could not be te sted. We unexpectedly obtained what appear to be the last available vials o f poliovaccine produced in 1955, In these vials, we detected and sequenced SV40 containing only one 72-bp enhancer in the regulatory region. The tissu e culture cytopathic test currently used in the United States to screen ora l poliovaccines was designed to detect rapidly proliferating SV40 virions c ontaining two 72-bp enhancers. We found that this test is not sensitive eno ugh to detect low amounts of the slow-replicating SV40 virions containing o ne 72-bp enhancer, This virus was easily detected in the same cells by immu nostaining and PCR, Twelve current vials of poliovaccines tested uniformly negative for SV40, suggesting that the precaution of preparing poliovaccine s from kidneys obtained from monkeys bred in isolated colonies prevented SV 40 contamination, Our data demonstrate that humans were exposed to SV40 vir uses with both one 72-bp enhancer and two 72-bp enhancers SV40 through cont aminated vaccines, Our data also suggest that instead of cytopathic tests, immunohistochemical and/or molecular studies should be used to screen polio vaccines for SV40 to completely eliminate the risk of occasional contaminat ion.