LIVE POLIO IRUS VACCINES

Effectiveness of Trivalent Vaccine in Immunizing Young Children
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with respect to heterotypic poliovirus types. In
our study, in which younger children, few of
whom had antibody to any type, were fed trivalent
vaccine, the highly infectious Type 3 seemed
to take over the susceptible sites in the intestinal
tract, almost completely crowding out the less
infectious Type 2 and suppressing also to some
extent Type 1. In older individuals, however,
a sizeable per cent may be expected already to
possess Type 3 (and/or Type 1) antibodies, so
that Type 2 has considerably less competition
and can establish itself and produce infection
and immunity in a higher per cent of individuals.
Actually, this is borne out in our study, in which
3 of 6 children already possessing Type 3 antibody
became infected with Type 2 virus on the
first feeding of trivalent vaccine. In contrast,
Type 2 infections occurred in only 2 of 29 triple
negatives on first exposure.
In a serological study, largely in adults, who
were fed trivalent vaccine containing 10'
6 2 TCD,,
of each of the three types, Cox et al. 2 have reported
that fourfold antibody rises occurred in
a very high per cent of individuals: >90 per cent
for Types 1 and 3, and 72 per cent for Type 2.
These are considered to be either primary or
"booster" effects due to vaccine takes with the
three types. However, if in this study only the
homotypic negatives for each type are considered,
and only conversions which can reasonably be
attributed to infection with the vaccine strains
(i.e. rises of from