LIVE POLIO IRUS VACCINES

Correlation of In Vitro Markers and Monkey Neurovirulence
37
the direct method of assay in monkeys is readily
available and accords well with the field behavior
of the strains in the human host.
SUMMARY
Base lines of normally expected neuroactivity
for each type of Lederle attenuated polioviruses
in consecutive lots of vaccine were established
by grading on a scale of 1 to 4 the spread of
lesions from the site of inoculation in 574 monkeys
inoculated intracerebrally with consecutive
lots of vaccine. Relative neuroactivity of a
vaccine lot was expressed as the sum of per cent
of animals showing grades 3 and 4.
The average incidence of combined grades 3
and 4 was 21 per cent for Type 1, 57 per cent for
Type 2, and 16 per cent for Type 3 virus. The
greater intracerebral activity of Type 2 virus,
which expresses itself in histopathologic lesions
and not in paralysis, is in marked contrast to its
clinical behavior in man, where it shows the
least "aggressiveness" of the three poliovirus
strains, and to its marker pattern which conforms
most closely to the one proposed for
avirulence.
All three Lederle poliovirus strains possess
the t- character, which appears to have the
best correlation with monkey avirulence, but
differ in their d and MS markers; Type 1 was
found to be d+ MS±; Type 2, d- + MS-; and
Type 3, d+ MS+. Some of the discrepancies in
the correlation between markers and monkey
virulence are discussed.
Since the value of d, t, and MS markers as criteria
of attenuation for polio vaccine viruses has
not been definitely established, residual neurovirulence
for monkeys remains the most acceptable
measure of strain attenuation.
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