LIVE POLIO IRUS VACCINES

58
Safety-Laboratory Evidence of Attenuation and Safety
be divided into two groups according to their
genetic stability after passage through the human
intestinal tract. One group comprises the relatively
stable Types 1 and 2 strains. Of this
group, the CHAT strain seems to show the highest
degree of stability of characters after human
passage, although it must be admitted that comparative
studies with the LSc, 2ab strain, which
showed a lesser degree of stability, have not
been carried out in the same laboratory. To the
second group belong all attenuated Type 3
viruses, none of which is stable after passage
through the human intestinal tract. This may
best be illustrated by the results shown in Table
8, where comparison is made of two markers-the
temperature and MS markers-applied to the
Type 1 CHAT virus and the Type 3 Wistar-Fox
strain. It will be observed that 19 out of 20
strains of Type 1 virus isolated from individuals
exposed to CHAT vaccine retained the original
temperature marker, and nine of the same strains
retained the MS marker-that is, reproduced
poorly on an established line of monkey-kidney
cells as compared to fresh monkey-kidney cells.
Totally different results were observed with the
Type 3 virus isolated from subjects exposed to the
Wistar-Fox strain. Only a small fraction of individuals
excreted virus resembling the original
vaccine strain in its characters. The remaining
strains changed their character. In addition, one
of the strains-Leon 12ab-has shown a lower
degree of immunizing capacity than the strains
representing the other two types of attenuated
virus. A search for an attenuated Type 3 virus
with characteristics better than those presently
available therefore seems to be indicated. Passage
of virus in tissue cultures maintained at
low incubation temperatures seems to be the
method of choice, and this procedure was extended
to investigations of possible attenuation of
strains representing Type 1 and Type 2 virus as
well. This work is now in progress and only preliminary
results can be presented.
The Type 1 virus employed in this study was
isolated by Fox and Gelfand from the stool of
an asymptomatic case in Louisiana (W-1 strain).
Following one passage in monkey-kidney cultures
at 37 ° C., the virus was transferred serially
through the same tissue-culture system kept initially
at 25 ° C. and later at 230 C. Following
15 passages at low temperatures, the virus was
plaqued at 230 C., and progeny of one plaque
were subjected to further serial passages at 23 °
C. Although the original strain grew equally
well on an established monkey-kidney cell line
and on fresh monkey kidney, growth of the
resulting cold variants on an established monkeykidney
line was inhibited after 13 passages at low
temperature, and the virus thus assumed the MS
character.
The cold strain called W-1 has been tested for
neuropathogenicity for Rhesus monkeys at three
different passage levels; the results are shown
in Table 9. Groups of monkeys were injected
intraspinally with undiluted tissue culture
medium following the technique described by
Melnick, and their central nervous system tissue
TABLE 8. COMPARATIVE STUDY OF Two MARKERS OF CHAT (TYPE 1) AND WISTAR-Fox (TYPE 3)
STRAINS EXCRETED BY VACCINATED SUBJECTS
VIRUS MARKER RATIO OF STRAINS* RETAINING ORIGINAL
MARKER OF VACCINE STRAIN
CHAT T/MPERATURE 19/20
MS 9/9
WISTAR-FOX TEMPERATURE 2/12
MS 1/12
* Denominator Number of strains tested.
Numerator =Number of strains showing the same marker as strain fed.