Some
polio vaccines prepared from 1954 to 1961 were contaminated with infectious SV40. It has been assumed that all
polio vaccines were SV40 free in the United States after 1961 and in other countries after 1962. Following a WHO requirement that was prompted by the detection of SV40 in some human
tumors, we conducted a multilaboratory study to test for SV40
polio vaccines prepared after 1961.
Vaccine samples from 13 countries and the WHO seed were initially tested by PCR. The possible presence of intact and/or infectious SV40
DNA in PCR-positive samples was tested by transfection and
infection of permissive
CV-1 cells. All results were verified by immunohistochemistry, cloning, and sequencing. All the
vaccines were SV40 free, except for
vaccines from a major eastern European manufacturer that contained infectious SV40. We determined that the procedure used by this manufacturer to inactivate SV40 in
oral poliovirus vaccine seed stocks based on heat inactivation in the presence of
MgCl2 did not completely inactivate SV40. These SV40-contaminated
vaccines were produced from early 1960s to about 1978 and were used throughout the world. Our findings underscore the potential risks of using primary monkey cells for preparing
poliovirus vaccines, because of the possible contamination with SV40 or other monkey viruses, and emphasize the importance of using well-characterized cell substrates that are free from adventitious agents. Moreover, our results indicate possible geographic differences in SV40 exposure and offer a possible explanation for the different percentage of SV40-positive
tumors detected in some laboratories.