Examination of poliovirus vaccine preparations for SV40 sequences

Oral poliovaccines derived from the strains developed by Sabin have been the basis of vaccination against poliomyelitis in the U.K. since 1962. Contamination of earlier materials with the monkey virus SV40, particularly inactivated Salk type poliovaccines, is well documented. Precautions have been in place for more than 30 years to prevent SV40 contamination of oral poliovaccines based on screening of donor animals and tests for SV40 infectivity. PCR was applied to examine all archived samples of oral poliovaccines available to us dating from 1966 to the present, including all vaccines used in the U.K. since 1980, for the presence of SV40 sequences. Of 132 materials examined, 118 were negative on initial testing and fourteen gave reactions which on further examination were attributed either to cross contamination during handling in the laboratory at National Institute for Biological Standards and Control (NIBSC) or to non-specific amplification. It was concluded that none of the samples contained SV40 sequences. The materials included 69 separate monovalent bulks of poliovirus type 1, 2 or 3 grown on monkey kidney cells from four different manufacturers and 74 bulks grown on human diploid cells from two manufacturers. One additional seed material from 1962 contained low levels of unique and characteristic SV40 sequences. The seed had been treated by the manufacturer to inactivate DNA viruses and tests by the manufacturer and at NIBSC failed to demonstrate the presence of infectious SV40 virus. Monovalent bulks prepared by the manufacturer from this seed were negative for SV40 sequences by PCR. The PCR studies provide no evidence of contamination of oral poliovaccines used in the UK with infectious SV40 and suggest that the steps taken to ensure the absence of infectious SV40 are satisfactory.