People rarely suffer from the same infectious disease twice. Reinfections normally occur, primarily (1) when the infectious agent exhibits antigenic plasticity such as with the common cold and influenza...
VACCINATION AND
IMMUNIZATION
Introduction
People rarely suffer from the same infectious disease twice. Reinfections
normally occur, primarily (1) when the infectious agent exhibits antigenic
plasticity such as with the common cold and influenza; (2) if the patient is
immunocompromised, due for example to immunosuppressive therapy or
immunological disorders; or (3) when a significant amount of time has passed
after the first infection. Alternatively, the patient may have failed to
eliminate the primary infection which remained latent and emerged later in a
modified of similar form as for example with herpes simplex (oral and genital
herpes), herpes zoster, (chickenpox) and HIV/AIDS.
Immunity against reinfection was recognized
long before the discovery of the causal agents of infectious disease.
Consequently, efforts were made towards developing treatment strategies that
could generate immunity to infection without the individual suffering the
infection. An early development was the attempted prevention of smallpox
(variola major) through the dermal inoculation of healthy individuals with
material taken from active smallpox lesions. Such treatments often produced
single localized lesions and commonly, but not always, protected the recipient
from contracting full-blown smallpox. The process became known as variolation and, unknown to its practitioners,
protected against the disease by changing the route of infection of the causal
organism from respiratory transmission to cutaneous. Unfortunately, occasional
cases of smallpox resulted from such practices and variolated individuals could
also (rarely) infect others, resulting in infection. Further developments
recognized that immunity developed towards one pathogen may be associated with
cross-immunity towards related infectious agents. Cowpox is a disease of cattle
that can be transmitted to humans. The symptoms are similar to those of
smallpox, but considerably less severe. Following the observation that
individuals exposed to cowpox were conferred protection against smallpox,
Edward Jenner substituted material taken from active cowpox (vaccinia) into the
variolation procedures. This conferred much of the protection against smallpox
that had become associated with variolation but without the associated risks. This
discovery, made over two centuries ago, became known as vaccination and heralded a new era in disease
control. The term vaccination was originally used to refer to prophylactic
measures that use living microorganisms or their products to induce immunity, but
the term is now used to refer to all immunization procedures.
Vaccination is used to protect
individuals against infection and also to protect communities against epidemic
disease. Such public health measures have met with spectacular success and in
instances where there is no reservoir of the pathogen other than in infected
individuals and survival of the pathogen outside the host is therefore limited,
vaccination has the potential to eradicate the disease permanently. This has
already been achieved for smallpox where the coordinated deployment of an
effective vaccine over many decades led to the eradication of this disease. The
global eradication of smallpox was endorsed by the World Health Assembly on 8
May 1980. Another candidate disease for global eradiation by vaccination is
poliomyelitis, where effective vaccination programmes have reduced the annual
incidence to fewer than 2000 cases. The virus persists, however, in India,
Pakistan, Afghanistan and Nigeria. The Global Polio Eradication Initiative,
spearheaded by the World Health Organization (WHO), Rotary International, the
U.S. Centers for Disease Control (CDC) and the United Nations Children’ s Fund
(UNICEF) is now actively working towards the eradication of this virus. It has
been calculated that eradication is a more cost-effective option than
containing the disease, as well as reducing morbidity associated with the
residual cases. The effectiveness of poliomyelitis vaccines is clearly
indicated by data shown in Figure 10.1.
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