LIVE POLIO IRUS VACCINES

Large-Scale Practical Trials, Use of Live Poliovirus Vaccine in USSR
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ticular, specific requirements, of which the
following are essential:
(1) the vaccine must be completely harmless;
(2) there should be a complete absence in
practice of untoward reactions to vaccination;
(3) the vaccine should be highly effective (it
is desirable that immunity should develop and
be maintained for several years in the overwhelming
majority of those vaccinated);
(4) there should be as a result of mass
vaccination a sharp reduction in the circulation
among the population of "wild" epidemic strains
of the causative agent of poliomyelitis;
(5) it should be easy to supply high-quality,
harmless vaccine for carrying out mass vaccination
of the population against poliomyelitis (on
any scale) and this means that there must be
an inexpensive and highly productive method of
preparing the vaccine and reliable procedures
for assessing its quality;
(6) there should be a simple and convenient
method of administering the vaccine which would
enable mass vaccination to be speedily carried
out.
Only if all these conditions are fulfilled will
it be possible to carry out the task of complete
immunization of the whole susceptible population
against poliomyelitis in the shortest possible
time.
The use of the prophylactic vaccine against
poliomyelitis, made by the method of Jonas Salk
(1951-1954) from formalin-inactivated virus, was
the first great achievement in the large-scale
specific prophylaxis of poliomyelitis. The Salk
vaccine to a considerable degree satisfies the
first three of the six requirements for poliomyelitis
vaccine listed above: it is harmless,
causes no reactions in practice, makes it possible
to reduce the incidence of paralytic forms of
the disease by 70-80 per cent, and sharply reduces
mortality. The technique of mass-producing
this vaccine was mastered in the Soviet
Union in 1956-1957 and during the last four
years about 24 million ml. of the vaccine have
been produced and used in the Soviet Union
(about 7 million persons have been inoculated).
The Salk vaccine made in the Soviet Union fully
satisfied the high standards demanded.
Nevertheless, the Salk vaccine is not sufficiently
effective. It leaves as many as 20-30
per cent of vaccinated persons unprotected
against paralysis. The immunity it establishes
is not complete and solid. In persons vaccinated
with the Salk vaccine the cells of the digestive
tract, the main "portals of entry" of the poliomyelitis
infection remain susceptible to infection
with the poliovirus, which is thus able to
multiply and to be transmitted to others. In
view of the further fact that it is in practice
impossible to ensure 100 per cent inoculation
of all children by injections of Salk vaccine
(because of the many reasons for exemption
based on contra-indications and the growing up
of new age groups, etc.), the polioviruses continuing
to circulate among the population indisputably
constitute a threat of new poliomyelitis
epidemics. During recent years several examples
of the occurrence of epidemics have accumulated
(in Israel, 1958, in a number of regions of the
United States of America, and in Hungary,
1959, etc.), despite the extensive campaigns for
large-scale injection of Salk vaccine previously
undertaken.
The production of a sufficiently immunogenic
inactivated vaccine and the laboratory testing of
its harmlessness require great efforts and considerable
expenditure; this slows down the rate
of development of production and makes it impossible
to ensure the timely immunization of
all who need it.
The very technique of intramuscular or subcutaneous
injection of the killed vaccine is too
cumbersome for giving repeated injections (a
minimum of four is needed) to many tens of
millions of people.
All this gave impetus to the search for new
methods of specific prophylaxis against poliomyelitis.
The most promising method proved
to be the use of live vaccines.
Experimental research and epidemiological
and clinical investigations (A. B. Sabin (1951-
1959), Koprowski et al., Cox, Cabasso et al., A.
A. Smorodintsev, M. P. Chumakov et al. (1959))
laid a good theoretical foundation for the elaboration
of suitable methods and for large-scale
testing of a live vaccine made from attenuated
poliovirus strains completely harmless for man.
The main advantages of immunization with
live vaccine over immunization with inactivated
vaccine are as follows:
(a) the convenience of oral administration
compared with injections;
(b) a saving of roughly one hundred-fold in
inoculation material compared with the killed