LIVE POLIO IRUS VACCINES

Discussion
411
DR. ANDERSON: My question is along the same
line as Dr. Melnick's. Dr. Sabin has referred to
a comparison with measles, pointing out that
there is a high incidence one year and a low
incidence the next. We know that, in a given
year, the period of spread of measles is concentrated
in a relatively short portion of that
year.
It may be that Dr. Sabin explained this and
I missed it, but I did not understand on what
basis he assumed that this reduction from 11
per cent of shedding of poliovirus down to .7 per
cent is something that was achieved by the immunization.
Has he any control data to show
that there may not have been seasonal variations
and that a decline of this sort could have
occurred independent of the immunization?
DR. SABIN: The only control data are that the
factors conducive to the dissemination of enteric
viruses continued, as is indicated by the fact
that the other enteric viruses were spreading to
the same extent.
The only thing that changed was the ratio of
polio to non-polio. The ratio of change is due
to the large number of resistant intestinal tracts
that are created by massive spread of polioviruses,
whether it be by nature or even more
extensively by man, as was done in Toluca.
Nature can do the same thing-if it spreads
Type 1 poliovirus extensively over a period of
six months in one year, the result is a natural
drop in Type 1 relative to other viruses the next
year. We were merely trying to do what nature
did, except in a much heavier and more extensive
manner.
DR. BODIAN: I think this is a very interesting
point and I wonder whether dissemination of
polioviruses in Querétaro is somewhat indicative,
although it can hardly serve as a completely
adequate control in this regard. Is there any
evidence that the wild polioviruses in the second
city were diminishing seasonally?
DR. SABIN: We had our hands full with Toluca.
Carrying out the serologic studies in Querétaro
is about as much as we were able to do.
DR. BODIAN: I should like to ask a question of
Dr. Ramos Alvarez, whose study I found most
interesting, and that is whether he has some
measure of the comparability of the two groups
with respect to the incidence of poliomyelitis,
that is, the unvaccinated and the vaccinated.
I do not believe I heard him mention any measure
of comparability or any factors which were
used to select those who received immunization.
DR. RAMOS ALVAREZ: The administration of
the vaccine, as I mentioned, was made on a
voluntary basis, and that was our method of
selection. I think that both groups are comparable,
since they belong to the same socioeconomic
level. Does this answer your question?
DR. BODIAN: What I really was getting at was
an independent measure of whether these two
groups are comparable with respect to their
probability of acquiring paralytic poliomyelitis.
For example, age and antibody status are factors
which would help in a comparison.
DR. RAMOS ALVAREZ: I think they are. I do
not have with me the information you requested
on the number of children in the different age
groups. However, I shall be glad to make the
data available later.
DR. MELNICK: Looking at Table 16*, of Dr.
Ramos Alvarez's paper, which indicates the maximum
effectiveness of the vaccine (from the number
of expected cases and the number of observed
cases), am I to understand that an effectiveness
of 80 per cent is what he feels he achieved in
this study?
DR. RAMOS ALVAREZ: I did not say it was an
effectiveness of 80 per cent, merely that there
was a striking difference between the groups.
DR. MELNICK: I should like an interpretation
of Dr. Ramos Alvarez's data, where he shows 187
expected cases and 35 observed. At face value
this comes out to about 80 per cent effectiveness.
Do I interpret the table correctly?
DR. RAMOS ALVAREZ: Yes, but we must take
one thing into consideration, that is, in Mexico
City the numbers are not comparable. We vaccinated
only 17 per cent of the estimated popula-
*see p. 398.