LIVE POLIO IRUS VACCINES

116
Safety-Field Evidence of Safety
fold or greater rises. In the absence of Type 2
excretion, these are interpreted as heterotypic
Type 2 responses to infection with Types 1 or 3
virus. The three who failed to show such heterotypic
rises all had high Type 2 titers (5512) to
start with; in two instances these may well have
already represented heterotypic responses since
the children were currently infected with Type 3
virus, prior to vaccine ingestion. Similar heterotypic
rises were observed in index children with
pre-vaccinal Type 3 antibodies who became infected
with other types.
DISCUSSION
The results presented indicate that in the vaccine
used, the infectiousness of the three types of
virus was quite different, and the use of a relatively
large dose (106TCD,,) did not completely
overcome interference between the three types.
The Type 3 component behaved as the dominant
one, and produced a considerably higher number
of infections than did Type 1, while Type 2,
under these circumstances, was greatly suppressed.
The impact of the second dose of vaccine,
one month after the first, was not great.
Probably if the interval between the two doses
had been longer, the second might have been
more effective.
In interpreting the results of this study, considerable
stress has been laid on controlling for
such factors as contact spread and heterotypic
antibody responses, which may be misleading
and may give false impressions of the actual
effectiveness of the trivalent vaccine per se. If
these qualifications are taken into consideration
in comparing our results with those of others
using the same trivalent vaccine, the differences
are actually not great.' 2, However, in
older children 1 and adults ' , 6 higher conversion
rates for Type 1, and particularly for
Type 2 have been reported. A factor which
might well be involved here is the immune status
TABLE 4. HETEROTYPIC TYPE 2 ANTIBODY RESPONSES TO INFECTION WITH POLIOVIRUSES TYPES 1
AND 3
NEUTRALIZING ANTIBODIES
NUMBER OF
POLIOVIRUS
CHILD SERUM 1 2 3 EXCRETION
11-1 pre