LIVE POLIO IRUS VACCINES

118
S afety-Field Evidence of S af ety
TABLE 5. RESULTS OF TESTS FOR THE THERMAL SENSITIVITY (T MARKER) OF 100 POLIOVIRUS
STRAINS EXCRETED BY VACCINEES AND THEIR CONTACTS
WEEKS TO VACCINATION
TOTAL
TYPE SOURCE 1-2 3-4 5-7 STRAINS
NO. NO. NO.
T- T+ T- T + T- T+ TESTED
15 VACCINEES 9 0 II 0 5 0 27
15 CONTACTS 4 0 7 0 8 0 24
3 VACCINEES 2 0 3 0 5
I CONTACT 1 0
16 VACCINEES 13 0 7 0 4 0 24
12 CONTACTS 13 0 5 0 2 0 20
dilution. One set of tubes was incubated at 36 °
and one at 40 ° C. The results were read at four
and seven days, and on the basis of the differences
in titer at the two temperatures, the strains
were designated as T-, T intermediate, or T+.'
In each test, a known T+ (Type 1 Mahoney)
and known T- (Type 1 LSc) strain were included
as controls.
The results for 101 strains, isolated at different
times after the vaccine was given, are
shown in Table 5. To our surprise, all isolates
of all three types behaved as T negative strains,
with differences of 5 logs or more between the
titer at 36 ° and 40 ° C.
In addition to the postvaccinal strains tested,
seven Type 3 poliovirus strains isolated from the
study group before the vaccine was administered
have all been shown to possess the T negative
character. It is entirely possible that these
strains represent a spread of Type 3 vaccine
strain from the nearby city of San José, where
this type had been used on a large scale for the
few weeks preceding our study.
Table 6 shows the results on the d markers of
40 of the same strains which were tested for T
markers. The three vaccine strains themselves,
as present in the preparation fed, were tested in
our laboratory, and all were classified as d intermediate.
Of the 40 strains excreted by vaccinees
and contacts, three had the d+ character, two
of these being Type 1 and one Type 3. A number
of other strains were like the vaccine strains,
d intermediate, and some were d-.
The results of tests with the two markers T
and d suggest so far that among susceptible
young Costa Rican children the vaccine strains
used were quite stable on human passage. The
results with Type 3, particularly with the T
marker, are at variance with the usual experience
with Type 3 vaccine strains and with Koprowski's
experience reported yesterday with the same
Type 3 Fox's strain. At present we do not have
an explanation of this discrepancy. As has been
shown ` the T marker does not always correlate
with monkey neurovirulence, nor with the d
marker.'2 Further tests, in order to compare the
T and d markers with monkey neurovirulence of
excreted strains, are now in progress.
ACKNOWLEDGMENTS
Grateful acknowledgment is expressed by the
authors to the following: Dr. José Manuel Quirce,
Minister of Health, and Dr. Oscar Vargas-Méndez,
Director General of Health, Costa Rica;