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A vaccine contains some kind of diminished form of a disease causing microbe. Depending on the characteristics of the microbe the scientists decide the form of the microbe to be used in a vaccine.

According to the National Institute of Allergy and Infectious Diseases, based on the state of the microbe in the vaccine, the vaccines are categorized as:

•Attenuated vaccines
•Inactivated vaccines
•Subunit vaccines
•Toxoid vaccines
•Conjugate vaccines
•DNA vaccines
•Recombinant vector vaccines


Live, Attenuated Vaccines

A vaccine prepared by using a live microbe, which has been weakened enough to not being able to cause a disease, is one of the most effective types of vaccines since the body is able to exposed to the real microbes it is able to produce effective antibodies that often last for a life time.

People with a damaged or weakened immune system – like those with HIV – cannot be given live vaccines because sometimes even the weakened state of the microbe can be strong enough for a person with a weak immune system and it can cause an infection.

There are also chances for a weakened live microbe in a vaccine to mutate and attain a more virulent form and cause disease.


Inactivated Vaccines

Inactivated vaccines contain dead microbes instead of weakened live ones. The microbes are inactivated by killing the disease-causing microbe with chemicals, heat, or radiation. Since a dead microbe cannot mutate into a virulent form, inactivated vaccines are safer than live vaccines. However, they are also less effective and need to given additional one or more booster doses for the effect to last.


Subunit Vaccines

Subunit vaccines include only a part of the microbe to include only the antigens that best stimulate the immune system. These types of vaccines are safer but are highly specific to only certain antigens. Once the scientists determine the best antigens that need to be used from the microbe, they can either cultivate the microbe in a laboratory or can create only the antigen molecules using a recombinant DNA technology.

The vaccine for hepatitis B is an example of subunit vaccine.


Toxoid Vaccines

Some types of bacteria cause illness due to the toxins they secrete. The scientists therefore use only the inactivated toxin produced by these bacteria to create a vaccine. Such “detoxified” toxins are called toxoids.

Vaccines against diphtheria and tetanus are examples of toxoid vaccines.


Conjugate Vaccines

Some harmful bacteria use an outer coating of poly-saccharides to disguise the antigens. The immune system in children is not mature enough to identify the disguise. The scientists therefore link antigens or toxoids from a microbe that an infant's immune system can recognize to the polysaccharides. The linkage helps the immature immune system react to polysaccharide coatings and defend against the disease-causing bacterium.

The vaccine that protects against Haemophilus influenzae type B is an example of a conjugate vaccine.


DNA Vaccines

The scientists are trying to analyze the genes of the microbes that cause diseases and create a vaccine that uses the genes that are a code for the actual disease causing antigens.

Genetic engineers have found that when the genes for a microbe’s antigens are introduced into the body, some cells will take up that DNA. The DNA then instructs those cells to make the antigen molecules. The cells secrete the antigens and display them on their surfaces. In other words, the body’s own cells become vaccine-making factories, creating the antigens necessary to stimulate the immune system.

A DNA vaccine is much more effective, safer, relatively easy and inexpensive to design and produce. Research is going on to find the best method of administering DNA vaccines into the human body.

So-called naked DNA vaccines consist of DNA that is administered directly into the body. These vaccines can be administered with a needle and syringe or with a needle-less device that uses high-pressure gas to shoot microscopic gold particles coated with DNA directly into cells. Sometimes, the DNA is mixed with molecules that facilitate its uptake by the body’s cells. Naked DNA vaccines being tested in humans include those against the viruses that cause influenza and herpes.


Recombinant Vector Vaccines

Recombinant vector vaccines are experimental vaccines similar to DNA vaccines, but they use an attenuated virus or bacterium to introduce microbial DNA to cells of the body. “Vector” refers to the virus or bacterium used as the carrier.

In nature, viruses latch on to cells and inject their genetic material into them. In the lab, scientists have taken advantage of this process. They have figured out how to take the roomy genomes of certain harmless or attenuated viruses and insert portions of the genetic material from other microbes into them. The carrier viruses then ferry that microbial DNA to cells. Recombinant vector vaccines closely mimic a natural infection and therefore do a good job of stimulating the immune system.

Attenuated bacteria also can be used as vectors. In this case, the inserted genetic material causes the bacteria to display the antigens of other microbes on its surface. In effect, the harmless bacterium mimics a harmful microbe, provoking an immune response.

Researchers are working on both bacterial and viral-based recombinant vector vaccines for HIV, rabies, and measles.