LIVE POLIO IRUS VACCINES

DISCUSSION
CHAIRMAN RHODES: These two papers are open
for discussion.
DR. DICK: I should like to ask Dr. Plotkin one
question. Could we have the age of the patients
in Table 8 of his presentation?
With regard to Dr. Gear's presentation it is
rather interesting to calculate the attack rates in
the Salk vaccinated and non-Salk vaccinated
individuals in Mauritius, and it appears that in
the Salk-vaccinated individuals the attack rate
was 6 per 100,000 while in the non-vaccinated individuals
it was 279 per 100,000, which suggests
an effectiveness of about 98 per cent.
DR. DULBECCO: 1 should like to comment on
the use of genetic markers, as mentioned in the
first report.
I think that I should be very careful in concluding,
based on the reported differences between
the original strain and some isolated
strains, that the latter were different strains not
derived from the first one.
In fact, it has been shown that, for instance,
a mutation from the inability to grow at 40 ° C.
to the ability to grow at 40 ° C., would very frequently
entail a change in neurovirulence.
Thus, the neurovirulent strains, if they are at
all related to the vaccine strain, differ from it
because of some mutation, which could be one
involving the T character. The isolated strains
may therefore not have the original T character,
even if they derive from the administered strain.
I should therefore caution, as I already pointed
out on the first day, to the fact that very many
markers, like the T, the d, and others, cannot
be used for identification of strains, because they
can change when a mutation occurs, which in
turn changes the neurovirulence.
The other remark is that I also would be
somewhat cautious about the use of the antigenic
marker as shown in the first report, because
mutations affecting some character, which for instance
modifies the speed of virus multiplication
in the cells in which the test is carried out, may
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modify the size of the plaques under antiserum,
even if there is no modification of the antigenic
character.
And I would suggest that this point be tested
by isolating mutants from the original strain, for
instance, by selection of some T+ at high temperature,
which would probably be neurovirulent,
and by testing for plaque formation under antiserum,
this mutant which certainly derives from
the original strain and the original strain itself.
DR. KOPROWSKI: These are very good points
made by Dr. Dulbecco, but of course we are dealing
with biological phenomena, which do not
describe absolute values but their relationships.
There are possibly two approaches to strains
isolated from such cases, either to do nothing, or
to try using the available markers to distinguish
them from other strains.
Now, as far as identity is concerned, I do not
think any one of us would identify a strain on the
basis of a T, d, MS, or any other such character.
We attempted to identify it on the basis of the
sero-differentiation test. From the data so far
available, we have not encountered a strain which
has been obtained after passage of this particular
virus through the human intestinal tract which is
not neútralized by the anti-CHAT serum.
The test suggested by Dr. Dulbecco is at present
being done. We have the very virulent T+
mutant of an attenuated strain obtained by Dr.
Lwoff after passage at 40 ° C. and we are checking
it against antiserum towards the attenuated
strain.
If these results are positive, I believe I would
be satisfied that the sero-differentiation test
identified excreted virus.
We have done a lot of work plaquing the original
virus and checking progenies of virus under
serum, and so far, out of 33 or 35 plaque progeny
tested, they are all neutralized by the anti-CHAT
serum.
DR. PLOTKIN: In reply to Dr. Dick's question,