LIVE POLIO IRUS VACCINES

Summary of the Conference
601
presence of what is presumed to be a hitherto
unrecognized virus in live vaccine and indeed in
some of the such seed strain preparations. This
agent, tentatively called the vacuolating agent,
can be identified, thus far, only in tissue-culture
preparations made from kidneys of monkeys of
the species Cercopithecus aethiops. The significance
of these findings for man is not known,
but it does illustrate the problem presented by
adventitious agents in the production and control
of live virus vaccine prepared on monkeykidney
cell cultures.
Field evidence of sajety
A fairly large series of new field trials involving
the use of the three different groups of
attenuated poliovirus strains, was reported by
24 groups of workers from 13 countries.
In the small trials involving not more than
500 people, close clinical supervision was available
on each and every participant. In the larger
trials, some of them nation-wide and ranging
from several hundred thousands to many millions
of participants, various methods of surveillance
techniques were followed. These clinical surveys
were designed particularly to detect illness
of the central nervous system among those who
had received the vaccine or among their intimate
associates. In almost all of these trials, including
the massive experience in the USSR, the opinion
was universal that untoward reactions were either
absent or insignificant and the so-called major
illness of poliomyelitis had not either directly or
indirectly been induced by infection with attenuated
poliovirus used as a vaccine. Nor had the
progeny of the vaccine virus induced harmful
effects as it spread in the local community.
It is to be pointed out that some of the trials
were carried out during the off season for poliomyelitis,
while in others sporadic or even epidemic
poliomyelitis was present coincidentally.
In the latter, cases of poliomyelitis are bound to
occur following, but not necessarily related to,
vaccination. For example, in one large trial
which was conducted at a time when a small
number of local cases of poliomyelitis were expected
in any event, a problem was encountered
in determining whether a few post-vaccinal infections
of the CNS were so related. The evidence
concerning these is as yet incomplete and
is being further investigated.
Spread of attenuated polioviruses
As was indicated in the report of last year's
Conference, the attenuated viruses which were
effective in producing a high level of serologic
responses spread readily to intimate family and
household associates, in somewhat the same manner,
but probably to a lesser degree than do
some of the wild polioviruses. This intra-familial
spread was reported as ranging from 40 to 80
per cent following feeding of infants, but may
be much less following feeding of older children,
and involved not only the susceptible contacts
but to a lesser degree those already possessing
homotypic antibodies. The degree to which these
attenuated polioviruses spread into the community
was measured in only a few instances.
Spread of this kind was not extensive.
In a large geographical area such as Czechoslovakia,
there was no evidence that the attenuated
strains of polioviruses spread beyond the
confines of the districts where they had been
administered-at least to a degree to indicate
that these adjacent populations were immunized.
In some of the reports from the Soviet Union,
it did appear that within urban groups of contacts
(so-called internal controls) whose members
were in relatively close contact with those
who had been vaccinated, some immunity was
acquired from this experience. More information
concerning spread is needed.
Immunization of Special Groups
Premature and newborn babies. The studies
carried out by several investigators on babies
given different oral vaccines have given results
which are substantially in agreement. It appears
that the vaccines can be given safely to premature
or full-term babies at or shortly after
birth and that excretion of virus occurs in a high
proportion of instances. The period of virus
excretion is, however, shorter in newborns than
in babies given vaccine at four months of age or
later and an antibody response is demonstrable
in only about half of the subjects and is often
delayed. The influence of high levels of antibody
derived from the mother is thought by some
to be an important factor in modifying or masking
the antibody response of newborn infants.
Pregnant women. Live poliovirus vaccine has
been given to several hundred pregnant women
without any untoward effects to the mothers.