LIVE POLIO IRUS VACCINES

Minnesota Studies With Oral Poliomyelitis Vaccine
171
DISCUSSION
Using the time interval of about three weeks
between feedings of the three types of poliovirus,
it appears, as judged by virus isolations, that
Type 1 replaced Type 2 and' Type 3 replaced
Type 1 very well (see Figure 1). The presence
of one virus is usually expected to interfere with
the establishment of another virus. Since there
was a time lapse of about one week between the
collection of the stool specimens and the later
feeding of another type, actual replacement is
not proven but is strongly suggested by the evidence
presented and is a most striking phenomenon.
The continued excretion of Type 3 poliovirus
in group B individuals, as shown by isolation
from the fifth and sixth stool specimens, is
in marked contrast to the small amount of continued
excretion of Types 2 and 1 poliovirus when
followed by the feeding of another type. This
long excretion may be a characteristic of the Type
3 vaccine strain and it is not known whether it
would have been well replaced if fed ahead of
Types 1 or 2. The possibility must also be considered
that the long continued excretion was observed
because no other virus was fed and therefore
there was less competition for lebensraum.
The absence of "wild" poliovirus in this community
was demonstrated by the absence of poliovirus
in the stool specimens collected prior to
feeding the vaccine strains and by the sequential
appearance of each type of poliovirus in stool
specimens only after each specific type had been
fed. Since only 16 cytopathogenic agents (adeno
Types 1 and 2) other than poliovirus were isolated
during this study the possible effect of enteric
viruses other than poliovirus in interfering
with the establishment of the vaccine strains of
poliovirus has not been measured.
The number of isolations shown for group B
individuals in Fig. 1 is not an accurate measure
of all of the persons who excreted virus. The
specimens were collected about two weeks after
virus feeding and it can be assumed that additional
group B individuals excreted virus for
shorter periods of time. This assumption is supported
by the previously published results showing
antibody response' which occurred for each
type in a larger proportion of the individuals
than those yielding virus in the stool specimen
collected two weeks after feeding. The value of
the isolation rates shown in Fig. 1 is therefore
limited to a comparison of the differences in
amounts of virus of each type available for community
spread rather than as a graphic representation
of the total number of excretors in the
community.
The difference in the amount of spread observed
with the three types is probably related to
the number of susceptibles in the community.l
Considering susceptibles as those without antibody,
the smallest number were susceptible to
Type 2, and Type 2 spread the least; the largest
number were susceptible to Type 3, and Type 3
spread the most. The difference in observed
spread of Types 1 and 3 was small, but as has
been pointed out, we have reason to believe that
the spread of Type 3 was less completely measured
in this study than were the spread of Types
1 and 2.
The presence of antibody in the pre-feeding
blood serum specimen does not always indicate
protection against the vaccine strains becoming
established in the intestinal tract (see Table 7).
It has previously been established by several investigators
that Salk vaccine is not effective in
preventing multiplication of poliovirus in the intestinal
tract. It also appears to be true that
Salk vaccine induced antibodies are not effective
in preventing multiplication of the vaccine
strains of poliovirus used in this study (see Table
7). In the absence of detectable poliovirus antibodies,
prior Salk experience does not markedly
reduce the virus isolation rates in either adults
or children. Differences in natural and Salk induced
antibodies cannot be determined, but it is
logical to assume that poliovirus antibodies are
more often natural in origin in adults than in
children. On the basis of this assumption the
differences in virus isolation rates observed in
adults and children who have detectable antibodies
and Salk experience is explainable (see
Table 7, last column). If the poliovirus antibodies
in the majority of the adults were natural
in origin the low rates of virus isolation which
were obtained (2.1 per cent to 7.1 per cent for the
various types) are to be expected; in a larger
proportion of the children showing antibodies
these antibodies can be assumed to be Salk induced
because the virus isolation rates are higher
(7.9 per cent to 43.1 per cent). It follows that