inherent in the production of live viral vaccines. It is this topic
that I am pleased to address here.
In 1798, Edward Jenner, an English physician advanced the use
of cowpox (vaccinia) virus for immunizing humans against
smallpox. He recognized that pathogens can behave differently
while infecting different species. Indeed, he theorized that the
vaccinia infection, which caused mild problems for cows, caused
more severe ailments in horses. Only after adapting to cows, did
vaccinia acquire limited infectivity for humans. The open sores
that humans developed were far less severe than those induced by
smallpox (variola) virus and essentially remained localized to the
site of inoculation. Moreover, contact with vaccinia virus caused
individuals to become virtually immune to the widespread
disease caused by the small-pox virus. The success of vaccination
is reflected in today's total elimination of smallpox as a disease.
Jenner's vaccination approach was followed in the twentieth
century by Pasteur's use of rabies virus grown in rabbit's brain,
and by Theiler's finding that he could reduce the effect of yellow
fever virus by growing it in chicken embryos.
These successes set the precedent for other scientists to attempt
to reduce the pathogenicity of other human and animal viruses by
inoculating them into foreign species. Although we now look
back with some disdain at the crudeness of early immunization
experiments - such as the 1938 injections of polio virus, grown in
mouse brains, into humans, most people, including scientists, are
unaware that we still use primary monkey kidney cells to produce
live polio virus vaccine. Likewise, dog and duck kidney cells
were used to make licensed rubella vaccines. Experimental
vaccines, grown in animal tissues and intended for human use,
were commonly tested in African monkeys, and it is likely that
many of these monkeys were released back into the wild. This
practice may have led to the emergence of primate diseases, some
of which could have been transmitted back to humans.
Large numbers of rural Africans were also chosen as test
recipients of experimental human vaccines.
In veterinary medicine, live viral vaccines have been widely used
in domestic pets and in animals destined to become part of the
food-chain. Undoubtedly, many cross-species transfer of viruses
have occurred in the process. Even today, more than ten foreign
species are used to produce currently licensed vaccines for cats
and dogs.
The general acceptance of the safety of cross-species produced
vaccines was supported in part by the generalization that there
are inherent restrictions to the interspecies spread of disease.
Thus, like vaccinia, most viruses are less hanmful, but others can
be far more dangerous after invading a foreign host. One
dramatic example is that of the human infection caused by the
herpes-type monkey B virus. This germ remains a rather
harmless invader of monkeys, but place it in humans, and
striking, severe, acute illness results which commonly ends in
death. Likewise, a modified horse-measles-virus (morbillivirus)