LIVE POLIO IRUS VACCINES

190
Safety-F'ield Evidence of Safety
190 Safety-Field Evidence of Safety
as others might grow better in human tissue.
The differences usually become manifest in vitro
only when the virus is grown under suboptimal
conditions.
The phenomena described may have a bearing
on the problem of selecting suitable live vaccine
strains. In principle the choice is now primarily
based on the results of neurovirulence
tests in monkeys on the assumption of a correlation
between monkey neurovirulence and pathogenicity
to man. To what extent this assumption
holds is not known, however. As a matter of
fact some strains isolated from paralytic human
cases produce only inapparent infections in monkeys,
while on the other hand strains obtained
from healthy children in interepidemic times not
infrequently turn out to be highly pathogenic to
monkeys. A systematic study of the differential
in vitro growth potential of strains of various
origin might shed some light on this important
problem.
I should like to add here that the designation
of a strain as t+ or t- really does not tell very
much about the characteristics of the strain.
This statement should be to a certain extent
qualified. Certainly, small variations in temperature
may cause rather great differences in the result,
and particularly the conditions in the culture,
such as bicarbonate content, should also be
taken into account.
1 believe also that the great variability in these
two respects observed in different strains of
viruses makes one hesitate a little to talk about
genetic markers. It seems to me that the RNA
viruses present problems that cannot be solved
with the classical methods we have learned to
use in the study of DNA genetics. For the time
being, I would prefer to talk about strain patterns,
behavioral patterns, rather than genetic
characteristics.
REFERENCES
1. Gard, S., Bottiger, M., and Lagercrantz, R.:
Vaccination with attenuated poliovirus Type
1, the CHAT strain. In: Live Poliovirus Vaccines,
First International Conference, Scient.
Pub. no. 44 of the Pan American Health Organization,
pp. 350-354, 1959.
2. Bottiger, Margareta, Gard, S., Kaijser, K.,
Lundstrom, R. To be published.
3. Bóttiger, Margareta. To be published.