LIVE POLIO IRUS VACCINES

92
Safety-Laboratory Evidence of Attenuation and Safety
sections of the cord or only one or two nuclei
of the brainstem sections affected.
A minimal lesion is one in which there is a
single focal infiltrate of one anterior horn or
one nucleus of the brainstem.
We feel that this descriptive grading system
has advantages in that it conforms to the trained
pathologists' impression, is simple to use and
does not convey an unwarranted impression of
precision. In dealing with a biologic system,
numerical quantitation tends to be non-reproducible.
RESULTS
The results obtained are shown in the following
tables. The number of plaque-forming
units (PFU) actually inoculated intramuscularly
are shown so that a comparison of the
strains may be made. Table 2 shows the results
after inoculation of the Type 1 strains
into Rhesus monkeys. As may be seen, monkeys
showed clinical signs of polio after inoculation
of 10 7.7 PFU, or greater, of the Koprowski or
the Lederle Type 1 strains. The first sign was
weakness of the leg in which the inoculum had
been placed. This occurred on the fifth or sixth
day after inoculation. Weakness gradually progressed
to partial or complete paralysis of the
affected leg, to partial or complete paralysis
of the other (left) leg as well. In this, as in
the initial study of these strains 1, many more
animals showed histologic evidence of polio than
the clinical signs would lead one to suppose.
With the Sabin Type 1 strain, no clinical signs
of poliomyelitis occurred; however, histologic
lesions of poliomyelitis were seen. Again, quantitative
differences are evident with the Sabin
strain; the lesions, in most of the animals, were
TABLE 3. NEUROVIRULENCE FOLLOWING INTRAMUSCULAR INOCULATION OF RHESUS MONKEYS WITH
TYPE 2 ATTENUATED POLIOVIRUS
* No. of monkeys having findings/No. inoculated.