Vaccine WORLD OF MICROBIOLOGY AND IMMUNOLOGY570•
only a few people die of rabies each year in the United States,
more than 40,000 die worldwide, particularly in Asia and
Africa. The less expensive vaccine will make vaccination far
more available to people in less developed nations.
The story of the most celebrated vaccine in modern
times, the polio vaccine, is one of discovery and revision.
While the virusesthat cause polio appear to have been present
for centuries, the disease emerged to an unusual extent in the
early 1900s. At the peak of the epidemic, in 1952, polio killed
3,000 Americans and 58,000 new cases of polio were reported.
The crippling disease caused an epidemic of fear and illness as
Americans—and the world—searched for an explanation of
how the disease worked and how to protect their families.
The creation of a vaccine for poliomyelitisby Jonas Salk
(1914–1995) in 1955 concluded decades of a drive to find a
cure. The Salk vaccine, a killed virus type, contained the three
types of polio virus which had been identified in the 1940s.
In 1955, the first year the vaccine was distributed, dis-
aster struck. Dozens of cases were reported in individuals who
had received the vaccine or had contact with individuals who
had been vaccinated. The culprit was an impure batch of vac-
cine that had not been completely inactivated. By the end of
the incident, more than 200 cases had developed and 11 peo-
ple had died.
Production problems with the Salk vaccine were over-
come following the 1955 disaster. Then in 1961, an oral polio
vaccine developed by Albert B. Sabin (1906–1993) was
licensed in the United States. The continuing controversy over
the virtues of the Sabin and Salk vaccines is a reminder of the
many complexities in evaluating the risks versus the benefits
of vaccines.
The Sabin vaccine, which used weakened, live polio
virus, quickly overtook the Salk vaccine in popularity in the
United States, and is currently administered to all healthy chil-
dren. Because it is taken orally, the Sabin vaccine is more con-
venient and less expensive to administer than the Salk vaccine.
Advocates of the Salk vaccine, which is still used exten-
sively in Canada and many other countries, contend that it is
safer than the Sabin oral vaccine. No individuals have devel-
oped polio from the Salk vaccine since the 1955 incident. In
contrast, the Sabin vaccine has a very small but significant rate
of complications, including the development of polio.
However, there has not been one new case of polio in the
United States since 1975, or in the Western Hemisphere since- Though polio has not been completely eradicated, there
were only 144 confirmed cases worldwide in 1999.
Effective vaccines have limited many of the life-threat-
ening infectious diseases. In the United States, children starting
kindergarten are required to be immunized against polio, diph-
theria, tetanus, and several other diseases. Other vaccinations
are used only by populations at risk, individuals exposed to dis-
ease, or when exposure to a disease is likely to occur due to
travel to an area where the disease is common. These include
influenza, yellow fever, typhoid, cholera, and HepatitisA and B.
The influenza virus is one of the more problematic dis-
eases because the viruses constantly change, making develop-
ment of vaccines difficult. Scientists grapple with predicting
what particular influenza strain will predominate in a given
year. When the prediction is accurate, the vaccine is effective.
When they are not, the vaccine is often of little help.
The classic methods for producing vaccines use biolog-
ical products obtained directly from a virus or a bacteria.
Depending on the vaccination, the virus or bacteria is either
used in a weakened form, as in the Sabin oral polio vaccine;
killed, as in the Salk polio vaccine; or taken apart so that a
piece of the microorganism can be used. For example, the vac-
cine for Streptococcus pneumoniae uses bacterial polysaccha-
rides, carbohydrates found in bacteria which contain large
numbers of monosaccharides, a simple sugar. These classical
methods vary in safety and efficiency. In general, vaccines that
use live bacterial or viral products are extremely effective
when they work, but carry a greater risk of causing disease.
This is most threatening to individuals whose immune systems
are weakened, such as individuals with leukemia. Children
with leukemia are advised not to take the oral polio vaccine
because they are at greater risk of developing the disease.
Vaccines which do not include a live virus or bacteria tend to
be safer, but their protection may not be as great.
The classical types of vaccines are all limited in their
dependence on biological products, which often must be kept
cold, may have a limited life, and can be difficult to produce.
The development of recombinant vaccines—those using chro-
mosomal parts (or DNA) from a different organism—has gen-
erated hope for a new generation of man-made vaccines. The
hepatitis B vaccine, one of the first recombinant vaccines to be
approved for human use, is made using recombinant yeast
cells genetically engineered to include the genecoding for the
hepatitis B antigen. Because the vaccine contains the antigen,
it is capable of stimulating antibodyproduction against hepa-
titis B without the risk that live hepatitis B vaccine carries by
introducing the virus into the blood stream.
As medical knowledge has increased—particularly in the
field of DNA vaccines—researchers have set their sights on a
wealth of possible new vaccines for cancer, melanoma, AIDS,
influenza, and numerous others. Since 1980, many improved
vaccines have been approved, including several genetically
engineered (recombinant) types which first developed during an
experiment in 1990. These recombinant vaccines involve the
use of so-called “naked DNA.” Microscopic portions of a
viruses’ DNA are injected into the patient. The patient’s own
cells then adopt that DNA, which is then duplicated when the
cell divides, becoming part of each new cell. Researchers have
reported success using this method in laboratory trials against
influenza and malaria. These DNA vaccines work from inside
the cell, not just from the cell’s surface, as other vaccines do,
allowing a stronger cell-mediated fight against the disease.
Also, because the influenza virus constantly changes its surface
proteins, the immune system or vaccines cannot change quickly
enough to fight each new strain. However, DNA vaccines work
on a core protein, which researchers believe should not be
affected by these surface changes.
Since the emergence of AIDS in the early 1980s, a
worldwide search against the disease has resulted in clinical
trials for more than 25 experimental vaccines. These range
from whole-inactivated viruses to genetically engineered
types. Some have focused on a therapeutic approach to helpwomi_V 5/7/03 10:59 AM Page 570