LIVE POLIO IRUS VACCINES

Discussion 375
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DR. Cox: I believe I stated in my paper that
there was no doubt that good antibody responses
were obtained against Types 1 and 3, or against
Types 1 and 3 combined. However, the poorest
conversion rates for Type 2 were obtained in the
triple negatives. We have made no claim that
all the Type 2 responses were entirely due to the
feeding of the Type 2 component. All we have
done is show the results we obtained serologically
on blood taken anywhere from four to seven
weeks later. These people are still available for
follow-up studies. Of course, it is quite difficult
to claim definitely that they may not have had
wild virus booster infections in the meantime.
If this is a problem, we know that if you feed
trivalent vaccine twice you fill in some of the
unfilled antibody gaps that were missed on the
first feeding. So we do not make the claim
that we can completely immunize the entire
population with a single feeding.
However, we are firmly convinced that the
trivalent vaccine has much merit. People get
broader antibody coverage across the board
when they are fed trivalent vaccine than when
they are fed monovalent vaccines. Like everything
else that has ever been done, improvements
are made as time goes on. At the present
time, we believe that trivalent vaccine fed on two
occasions would do a better job than trivalent
vaccine fed a single time. However, this is not
the time or place to argue about this; I believe
this discussion can be carried on at some other
time.
DR. VAN ROOYEN: I have listened with great
interest to the criteria of conversion as indicated
by antibody response. When an individual is
vaccinated against smallpox, nobody carries out
antibody titration to determine whether or not
the individual is successfully vaccinated. It is
customary to observe the lesion and to satisfy
oneself whether or not the individual is protected
according to the dermatological response.
In poliomyelitis, I believe that infection of the
gut takes place, and subsequently, the receptivity
or otherwise of the intestinal wall decides
whether or not that particular section of gut has
been immunized against wild virus.
Speaking for myself, I feel that the capacity
of the intestine to reject wild virus is a better
yardstick of immunity than the serological antibody
response, particularly in adults.
The other point I should like to make is that
Dr. Cox probably is going to receive much advice
on how to prepare his own vaccine and to
this I should like to add still more. As far as
Nova Scotia, Canada, is concerned, the three
strains of Cox virus as now present in the vaccine
are well balanced to suit our area, where a
high percentage of Type 2 antibody prevails
among the population. Therefore, I feel that
these agents should be harnessed and fed together
as a team, so that each virus type could be encouraged
to proliferate in the gut, in approximately
the same relative proportion as its
epidemiological distribution. Thus, the best advice
I can give Dr. Cox is to leave the present
Cox vaccine unchanged.
The third point I wish to make is to ask Dr.
Langmuir if he would deal with another unique
situation, namely, if he could explain why some
25 per cent of a total of 147 paralytic cases of
poliomyelitis, due to Type 1 in Newfoundland,
occurred in young children who had received
three doses of Salk vaccine.
DR. LANGMUIR: We had a large number of
triple vaccinated cases and even a small proportion,
roughly 3 per cent of our reported cases,
quadruply vaccinated in this country. Except
for the Massachusetts story, these have been concentrated
in crowded slums, isolated, ethnically
distinct groups, the groups consistently throughout
the country least well vaccinated. I suspect
that the same thing was true in Newfoundland,
that you had a sharp epidemic in unvaccinated
population groups if it was a Type 1 epidemic.
If we take surveys, such as in the Des Moines
epidemic, which was studied most intensively,
we can relate the cases and get specific attack
rates by vaccine doses and by social economic
class. From this we can get corrected estimates
of effectiveness which indicate that Salk vaccine,
three or more doses, as given over the past four
years in this country, is in the range of 80 per
cent effective. This I believe is a conservative
estimate.
I cannot yet give a carefully evaluated estimate
of the effectiveness of four doses, but in
my judgment, I believe it will come out in the
range of 90 per cent or more.