LIVE POLIO IRUS VACCINES

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Efficacy-Field Evidence
ference between three types of poliovirus. For
instance, prevalent dissemination of Type 2
poliovirus is frequently observed in interepidemic
periods to the detriment of latent immunization
with Types '1 and 3, which leads to
the accumulation of menacingly large numbers
of susceptibles to poliovirus Types 1 and 3; and,
on the contrary, subsequent decrease of Type 2
virus circulation is possible in epidemic years
when more active Type 1 or Type 3 strains are
prevalent. Irregular alternations of poliovirus
types prevalent in different periods, with constant
separation of more pathogenic mutants,
increase the chaotic state of latent immunization.
The history of poliomyelitis for the past 10 years
shows that poliomyelitis epidemics can occur in
any country regardless of hygienic standards of
living and of climatic conditions. Consequently,
nobody can any longer depend on "favors from
nature" in the form of very unreliable (because
of its irregularity) latent immunizations with
"wild" poliovirus strains.
Therefore, it is only with the aid of most
extensive measures for rapid creation of artificial
immunity in the entire susceptible population
that the menace of poliomyelitis epidemics can
be averted.
For this purpose it is necessary to have an
effective, readily available, completely safe
areactogenic vaccine presenting no difficulties in
its most wide application.
Trials of live poliovirus vaccine from Sabin's
strains carried out in a number of countries,
particularly during 1959 in our country on a
large scale, furnished convincing proof that it
is the live vaccine that is most suitable for mass
immunization.
A great advantage of the live vaccine, compared
to the killed Salk vaccine, lies in the
possibility to create in the vaccinated more or
less complete immunologic resistance of tissue
cells in the "portals of entry" of infection, that
is, in the walls of the alimentary tract including
the pharyngeal circle. This will probably cause
a sharp reduction in the circulation of wild epidemic
strains of poliovirus among the population
as a result of repeated immunization with
attenuated viruses. In this way an important basis
for eradication of poliomyelitis epidemics is being
created.
One of the most frequent objections raised
against the use of live poliovirus vaccine is the
fear of possible reversion of attenuated poliovirus
strains to pathogenic properties which may
occur as a result of selection in the human
intestinal tract of virulent virus particles and of
repeated passages through the susceptible
population.
There are many facts available at present
testifying to a sufficient stability of the genetic
properties of Sabin's attenuated strains, which
persists during several passages through the
human body. There are no theoretical and experimental
grounds, however, which would make
it possible to exclude entirely the possibility of
degeneration occurring in the vaccine strains under
the effect of their propagation under inappropriate
conditions, or due to a long-term selection
of virulent virus particles and their multiplication
on passages through susceptible human beings.
In connection with this, we put forward in
1958-1959 the principle of mass and simultaneous
immunization with live poliovirus vaccine
covering the territory of a whole district, city,
or region, so as to ensure maximum involvement
by immunization in the shortest possible time,
and to minimize the possibility of a long-term
circulation of any poliovirus strains among susceptible
population and of increase in their
neurovirulence during serial passages in children.
In carrying out mass vaccinations during 1959-
1960 we closely adhered to this principle.
The necessity to use live vaccine for immunization
of many millions of persons required searching
for the most convenient form of vaccine
preparation so as to minimize labor spent on vaccine
distribution, and to simplify maximally the
procedure of oral immunization of people.
Under our conditions, considering the unusually
large scope and narrow time limits set
for oral immunization in 1960, it would be
extremely difficult to secure the dilution of
thawed liquid vaccine and its distribution dropwise
throughout the country by the local medical
staff in the field (as it was done in 1959).
In this connection the Institute for Poliomyelitis
Research, in cooperation with the Research Institute
of confectionery industry, has elaborated
a new presentation of the live vaccine incorporated
into pasty candies and later into dragéecandy.
This was based on the data (G. P. Fleer,