Vaccines work by exposing the immune system to antigens from viruses or bacteria in order to induce immunity. They are composed of weakened or killed pathogens, or purified components or proteins from pathogens. First generation vaccines use whole pathogens, while second generation use purified subunits or components. Third generation DNA vaccines contain DNA that codes for pathogen antigens. Vaccines trigger both antibody and cellular immune responses to provide protection. Different types include live attenuated, inactivated, toxoid, subunit, and conjugate vaccines. Adjuvants can be added to vaccines to enhance the immune response. Herd immunity occurs when a sufficient percentage of a population is immune, indirectly protecting those who are not immune.

1. VACCINES

2. Vaccines
• Vaccines are composed of treated microorganisms or viruses,
chemically altered toxins, or chemical parts of
microorganisms.
• Such vaccines work by mimicking a “natural” infection.
• By exploiting the immune system’s ability to recognize
antigens and respond with antibodies and lymphocytes, a
vaccine triggers a primary antibody response.
• Vaccines may be administered by injection, oral consumption,
or, as is used for some influenza vaccines, nasal spray.

3. • It is a preparation of the causative agent of a
disease, specially treated for use in
vaccination, in order to induce or increase
immunity.
• Typically contains an agent that resembles a
disease-causing microorganism, and is often
made from Weakened or killed
microorganisms or subunits (purified protein
subunits, polysaccharides).

4. “Generations” of Vaccines
• The first generation of vaccines were
developed by using the whole bacterium, virus,
or toxin as the antigen.
• Such vaccines are the closest to the natural
pathogens and, therefore, they generate the
strongest immune response. Often, the person
vaccinated will have lifelong immunity.

5. Attenuated Vaccine
• An attenuated vaccine is a vaccine created by reducing
the virulence of a pathogen, but still keeping it viable (or
"live”) so that it becomes harmless or less virulent.
• Deliberate weakening, called attenuation, can lead to the
production of live attenuated vaccines of attenuated
viruses.
• There are many attenuated viral vaccines. The Sabin oral
polio vaccine, as well as the measles, mumps, and
chickenpox vaccines, contain attenuated viruses.
• To avoid multiple injections of immunizing agents, it is
sometimes advantageous to combine vaccines into a single-
dose vaccine. The measles-mumps rubella (MMR) vaccine
is one example.

6. Inactivated Vaccines
• Another strategy for preparing vaccines is to
“kill” the pathogen.
• These vaccines are relatively easy to produce
because the pathogen is killed by simply using
certain chemicals, heat, or radiation.
• However, the inactivation process alters the
antigen so it produces a weaker immune
response.

7. • The Salk polio vaccine and the hepatitis A
vaccine typify such preparations of inactivated
whole viruses.
• For protection from diseases like hepatitis A,
booster shots are required to maintain
immunity for long periods of time. In the case
of influenza, the virus changes genetically
from year to year.

8. • Some whole organism (bacterial) vaccines are
used for short-term protection. For instance,
bubonic plague and cholera vaccines are available
to limit an epidemic.
• In these cases, the immunity lasts only for several
months because the material in the vaccine is
weakly antigenic. Compared to attenuated
vaccines, inactivated vaccines are safer as they
cannot mutate and therefore cannot cause the
disease in a vaccinated individual.
Bacterial Vaccines

9. Toxoid Vaccines
• For some bacterial diseases, such as diphtheria
and tetanus, a bacterial toxin is the main cause
of illness. So, a third immunization strategy is
to inactivate these toxins and use them as a
vaccine. Such toxins can be inactivated with
formalin, and the resulting inactivated toxin is
called a toxoid.
• Immunity induced by a toxoid vaccine allows
the body to generate antibodies and memory
cells to recognize the natural toxin.

10. • Single-dose vaccines include
diphtheriapertussis-
• tetanus vaccine (DPT) and the newer
diphtheria-tetanus-a-cellular pertussis (DTaP)
vaccine.

11. Second Generation Vaccines
• To minimize the risks of vaccination, second
generation vaccines have been developed that
contain only a fragment of the bacterium or
virus.
• These subunit and conjugate vaccines
generate acquired immunity.

12. Subunit Vaccines
• Unlike the whole agent attenuated or inactivated
vaccines, the strategy for a subunit vaccine is to
have the vaccine contain only those parts or
subunits of the antigen that stimulate a strong
immune response.
• These subunits may be epitopes.
• For example, the subunit vaccine for
pneumococcal pneumonia contains 23 different
polysaccharides from the capsules of 23 strains of
Streptococcus pneumoniae.

13. • One way of producing a subunit vaccine is to
use recombinant DNA technology, where the
resulting vaccine is called a recombinant
subunit vaccine.
• The hepatitis B vaccine (Engerix-B) is an
example. Several hepatitis B virus genes are
isolated and inserted into yeast cells, which
then synthesize the antigens. These antigens
are collected and purified to make the
vaccine.

14. • Adverse reactions to such subunit vaccines are
very rare because only the important subunits
of the antigen are included in the vaccine.
• These subunits cannot produce disease in the
person vaccinated.

15. Conjugate Vaccines
• Since the capsular polysaccharides by themselves are
not strongly immunogenic, the strategy here is to
conjugate (attach) capsular polysaccharides to tetanus
or diphtheria toxoid, which will stimulate a strong
immune response.
• The result is the Hib vaccine, which has been a critical
factor in reducing the incidence of Haemophilus
meningitis.

16. • In 2005, a conjugate vaccine (MCV4) was licensed
for a meningococcal vaccine against meningitis
caused by Neisseria meningitidis.

17. DNA Vaccines
• DNA vaccines are third generation vaccines.
• They contain DNA that codes for specific
proteins (antigens) from a pathogen.
• These investigational DNA vaccines consist of
plasmids engineered to contain one or more
protein-encoding genes from a viral or bacterial
pathogen. They are simply injected in a saline
solution.

18. • Unlike replicating viruses or live bacteria, plasmids are
not infectious or replicative, nor do they encode any
proteins other than those specified by the plasmid
genes; someone vaccinated with a DNA vaccine could
not contract the disease.
• They are relatively easy to construct and produce, and
the vaccines are more stable than conventional vaccines
at low and high temperatures, making shipping of these
vaccines easier.

19. Immune Responses by Vaccines
First generation vaccines are whole-organism vaccines – either
live and weakened, or killed forms. Live, attenuated vaccines,
such as smallpox and polio vaccines, are able to induce killer T-
cell responses, helper T-cell responses and antibody immunity.
However, attenuated forms of a pathogen can convert to a
dangerous form and may cause disease in immuno-
compromised vaccine recipients (such as those with AIDS).
While killed vaccines do not have this risk, they cannot generate
specific killer T cell responses and may not work at all for some
diseases.

20. Subunit
Second generation vaccines were developed to reduce the risks
from live vaccines. These are subunit vaccines, consisting of
specific protein antigens (such as tetanus or diphtheria toxoid
or recombinant protein components (such as the hepatitis B
surface antigen). They can generate TH and antibody responses,
but not killer T cell responses.
DNA
DNA vaccines are third generation vaccines. They contain
DNA that codes for specific proteins (antigens) from a
pathogen. The DNA is injected into cells, whose "inner
machinery" uses the DNA to synthesize the proteins. Because
these proteins are recognized as foreign, when they are
processed by the host cells and displayed on their surface, the
immune system is alerted, which then triggers immune
responses.

21. Adjuvants
An adjuvant is a chemical substance that can be added to a vaccine
in order to enhance the immune response to the vaccine. There are
three types of adjuvants.
• Aluminum Hydroxide and Aluminum Phosphate (Alum)
Alum is an inorganic salt that binds to proteins and causes them to
precipitate. Whenever the alum/vaccine complex is injected into
the body, it slowly dissolves, releasing the vaccine.
• Bacterial extracts can be added, which enhances the immune
response. Alum is the only adjuvant approved for use in humans.
• Freund’s Adjuvant, the vaccine is suspended in oil droplets.
When injected into the body, the vaccine slowly diffuses out of
the oil drop. Bacterial antigens can be added in order to enhance
the immune response. It is not used in humans because of risk of
severe inflammation.

23. Herd immunity (also called herd effect, community immunity,
population immunity, or mass immunity) is a form of indirect
protection that applies only to contagious diseases. It occurs when a
sufficient percentage of a population has become immune to an
infection, whether through previous infections or vaccination,
thereby reducing the likelihood of infection for individuals who
lack immunity.
Once the herd immunity has been reached, disease gradually
disappears from a population and may result in eradication or
permanent reduction of infections to zero if achieved worldwide.
Herd immunity created via vaccination has contributed to the
reduction of many diseases.