Page 294 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 36 Viral Vaccines
(3) Varicella-zoster immune globulin (VZIG) is used in
the prevention of disseminated zoster in people who may
Herd immunity (also known as community immunity)
have been exposed to the virus and who are immunocom-
occurs when a sufficiently large percentage of the popula-
promised. The preparation contains a high titer of antibody
tion (the “herd”) is immunized so that an unimmunized
to varicella-zoster virus and is obtained from humans to
avoid hypersensitivity reactions.
nity to occur, the vaccine must prevent transmission of the
(4) Vaccinia immune globulins (VIG) can be used to treat
virus as well as prevent disease. For example, the live,
some of the complications of the smallpox vaccination.
attenuated polio vaccine can provide good herd immunity
(5) Immune globulins (IGs) are useful in the prevention individual is protected (see Chapter 33). For herd immu-
because it induces intestinal IgA, which prevents poliovirus
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(or mitigation) of hepatitis A or measles in people who
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from replicating in the gastrointestinal tract and being
may have been exposed to these viruses. IGs are commonly
transmitted to others. However, the killed polio vaccine
used prior to traveling to areas of the world where hepatitis
A virus is endemic. IGs contain pooled serum obtained
not produced, and immunized individuals (although pro-
from a large number of human volunteers who have not
been hyperimmunized. The effectiveness of IG is based on
tected from poliomyelitis) can still serve as a source of
antibody being present in many members of the pool.
poliovirus for others.
PEARLS
Active Immunity • The main advantage of passive immunity is that it provides
naturally when IgG is transferred from the mother to the fetus
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across the placenta and when IgA is transferred from the
• Active immunity can be elicited by vaccines containing killed
mother to the newborn in colostrum.
viruses, purified protein subunits, or live, attenuated (weak-
ened) viruses.
immediate protection. The main disadvantage is that it does
• In general, live viral vaccines are preferable to killed vac-
not provide long-term protection (i.e., it is active only for a few
cines for three reasons: (1) they induce a higher titer of anti-
weeks to a few months).
body and hence longer-lasting protection; (2) they induce a
broader range of antibody (e.g., both IgA and IgG, not just
A virus, hepatitis B virus, and varicella-zoster virus are
IgG); and (3) they activate cytotoxic T cells, which kill virus-
effective.
infected cells.
• There are some potential problems with live viral vaccines,
immune globulins and a viral vaccine. This provides both
the most important of which is reversion to virulence.
immediate as well as long-term protection. For example, pro-
Transmission of the vaccine virus to others who may be immu- • Passive–active immunity consists of administering both
tection against rabies in an unimmunized person who has
nocompromised is another concern. Also there may be a sec-
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been bitten by a potentially rabid animal consists of both
ond, unwanted virus in the vaccine that was present in the cells
rabies immune globulins and the rabies vaccine.
used to make the vaccine virus. This second virus may cause
adverse effects.
• Live viral vaccines should not be given to immunocompro-
mised individuals or to pregnant women.
• Herd immunity is the protection of an individual that
• Vaccines grown in chick embryos, especially influenza vaccine,
results from immunity in many other members of the popu-
lation (the “herd”) that interrupts transmission of the virus to
should not be given to those who have had an anaphylactic
the individual. Herd immunity can be achieved either by
reaction to eggs.
high percentage of the population. Herd immunity is
Passive Immunity
unlikely to be achieved by passive immunity because,
• Passive immunity is immunity acquired by an individual by active immunization or by natural infection of a sufficiently
although antibodies can protect the individual against
the transfer of preformed antibodies made in either other
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spread of virus through the bloodstream, they are unlikely
humans or in animals. These antibody preparations are often
to prevent viral replication at the portal of entry and conse-
called immune globulins. Passive immunity also occurs
quent transmission to others.

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