The seed is the original polio vaccine material created by Drs. Sabin and Salk that would be used to make all subsequent polio vaccines for decades.  The creation of these seeds explains how they became contaminated with SV40.

As you read this, keep in mind that rhesus monkeys are known carriers of SV40 and African Green monkeys can also be a carrier, but less frequently.  The term “passaging” is also important.  It means introducing a virus into a living being (i.e. by oral, nasal, inoculation) and then harvesting that virus after it has multiplied and adapted itself to that host.  Passaging can make a virus less virulent.

In the 1950s, scientists like Doctors Jonas Salk and Albert Sabin had isolated the poliovirus strains to make vaccines.[1]  Dr. Salk’s strains would be inactivated with formaldehyde and injected into children.  Dr. Sabin’s strains would be attenuated or weakened by transferring or passaging[2] the live viruses through different host cells and then fed to children orally.

Because his goal was to create a live attenuated vaccine, Dr. Sabin had to isolate the poliovirus strains and then passage the strains through a myriad of host cells in order to attain the right virulence—strong enough to illicit an immune response, but weak enough so as to not cause polio in the recipient.  Sabin’s oral polio vaccine  (OPV) is a trivalent vaccine and was, therefore, comprised of three types – Type I, II, and III.

For example, Type I has the following lineage: In 1941, Drs. Francis and Mack isolated the Mahoney poliovirus “from the pooled feces of three healthy children in Cleveland.”[3]  Dr. Salk then subjected the strain to passages through fourteen living monkeys and two cultures of monkey testicular cultures.[4]  In 1954, the strain (now called Monk14 T2) was given to Drs. Li and Schaeffer who subjected the virus to nine more passages through monkey testicular cultures.[5]  Next, the strain (now called Monk14 T11) underwent fifteen more passages in monkey testicular cultures, eighteen passages in monkey kidney cells, two passages through the skin of living rhesus monkeys, and additional passages through African Green monkey skin and monkey kidney cell cultures.[6]  This strain was now called MS10 T43 or LS-c.  In 1956, Dr. Sabin took this virus and passaged it through seven cultures of African Green Monkey kidney cells.[7]  That same year, the pharmaceutical company, Merck, Sharp & Dohme, passed the strain (now called LS-c, 2ab/KP₂) through a rhesus monkey kidney cell culture.[8]  The resulting material was called Sabin Original Merck (SOM) and was provided to Lederle in 1960 as the seed material to manufacture its polio vaccine.  Types II and III were created in a similar fashion.[9]

References:

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