Vaccination (continued)
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prevaccine highs. The dream of eradication has been fulfilled
in the case of smallpox, and polio has nearly been eradicated
(although isolated polio outbreaks still occur in a handful of
countries).
Implicit within Jenner's method of vaccinating against
smallpox was the recognition of immunologic cross-reactivity
together with the notion that protection can be obtained
through active immunization with a different, but related,
live virus. It was not until the 1880s that the next immunizing
agents, vaccines against rabies and anthrax, were introduced
by Pasteur. Two facts of his experiments on rabies vaccines are
particularly noteworthy.
First, Pasteur found that serial passage of the rabies agent in
rabbits resulted in a weakening of its virulence in dogs. For the
next 100 years, Pasteur's empirical approach for attenuating the
virulence of a live virus by repeated passages in cells of species
different from the natural host remained the principal empirical
method for developing attenuated-live-virus vaccines. During
multiple passages in an animal or in tissue culture cells, muta-
tions accumulate as the virus adapts to its new environment.
These mutations adversely affect virus reproduction in the
natural host, resulting in lessened virulence. Only as the molec-
ular basis for virulence has begun to be elucidated by modern
biologists has it become possible to deliberately remove the
genes promoting virulence so as to produce attenuated viruses.
Second, Pasteur demonstrated that the rabies virus retained
immunogenicity even after its infectivity was inactivated by for-
malin and other chemicals, thereby providing the paradigm for
one class of noninfectious virus vaccine, the killed-virus vaccine.
Attenuated-live and inactivated vaccines are the two broad
classifications for vaccines (see table). Anti-idiotype antibody
vaccines and deoxyribonucleic acid (DNA) vaccines represent
innovations in inactivated vaccines. Recombinant-hybrid virus-
es are novel members of the live-virus vaccine class produced
by genetic engineering.
Fig. 1: Vaccination protects populations through community or herd immunity. When
enough members of a community are immunized against a particular contagious disease,
they are protected against that disease. (Top) A community in which no one is immunized;
thus, an outbreak is liable to occur. (Middle) Some members of a community are immunized,
but the amount is not enough to result in community immunity. (Bottom) Most members
of a community are immunized, thereby providing protection to the population. (Credit:
National Institute of Allergy and Infectious Diseases)
Table 1 - Some of the major types of vaccines that provide immunization
Vaccine type Disease
Attenuated-live Measles, Mumps, Polio (oral, Sabin), Rotavirus, Rubella, Tuberculosis (BCG), Varicella (chicken pox),
Yellow fever
Inactivated killed Cholera, Hepatitis A, Influenza (injection), Polio (IPV, Salk), Rabies, Typhoid, Whooping cough
(whole-cell pertussis)
Subunit/Conjugate Haemophilius influenzae type B, Hepatitis B (HepB), Human papillomavirus, Meningococal diseases,
Pneumoccoccal diseases
Toxoid (inactivated toxin
protein)
Diphtheria, Tetanus
