a) Live, attenuated vaccines

These vaccines contain a modified live germ that has been weakened so that it cannot cause disease. Because a live, attenuated vaccine is the closest thing to a natural germ that causes infection, these vaccines stimulate the immune system in almost a similar manner to the pathogen. Therefore, live, attenuated vaccines elicit strong immune response and often confer lifelong immunity with only one or two doses. For example, the yellow fever vaccine.

Depending on the type of organism and the modification (attenuation), live and modified vaccines can be easy or difficult to create. Vaccines against measles, mumps, and chickenpox, for example, are made by growing in hens egg making them non-virulent when administered in humans. Viruses are simple microbes containing a small number of genes, and scientists can therefore more readily control their characteristics. Viruses often are attenuated through a method of growing generations of them in cells in which they do not reproduce very well.

Live, attenuated vaccines are more difficult to create for bacteria. Bacteria have thousands of genes and much harder to control than viruses. Scientists working on a live vaccine for a bacterium, however, might be able to use recombinant DNA technology to remove several key genes. This approach has been used to create a vaccine against the bacterium that causes cholera, Vibrio cholerae. Live vaccines require refrigeration. An example of live, attenuated bacteria vaccine is BCG.

b) Inactivated vaccines

These types of vaccines are made by killing the disease-causing germ with chemicals, heat, or radiation. Such vaccines are more stable. Most inactivated vaccines, however, stimulate a weaker immune system response than do live vaccines. So it would likely take several additional doses, or booster shots, to maintain a person’s immunity. This could be a drawback in areas where people don’t have regular access to health care and can’t get booster shots on time.

Inactivated vaccines usually don’t require refrigeration, and they can be easily stored and transported in a freeze-dried form, which makes them accessible to people in resource poor settings.

c) Subunit vaccines

These types of vaccines include only the antigens that best stimulate the immune system. In some cases, these vaccines use epitopes—the very specific parts of the antigen that antibodies or T cells recognize and bind to. Subunit vaccines can contain anywhere from 1 to 20 or more antigens. Identifying which antigens best stimulate the immune system to include them in a vaccine is a tricky, time-consuming process. Once scientists identify the most appropriate antigens to include in a subunit vaccine, they can either grow the microbe in the laboratory and then use chemicals to break it apart and gather the important antigens or manufacture the antigen molecules from the microbe using recombinant DNA technology. Vaccines produced this way are called “recombinant subunit vaccines.”

d) Toxoid vaccines

These vaccines are made by inactivating toxins by treating them with formalin, a solution of formaldehyde and sterilized water. Such “detoxified” toxins, called toxoids, and are safe for use in vaccines. Toxoid vaccines are made for bacteria that secrete illness-causing toxins, or harmful chemicals. When the immune system receives a vaccine containing a harmless toxoid, it learns how to fight off the natural bacterial toxin, which causes an illness. The immune system produces antibodies that lock onto and block the toxin. Vaccines against diphtheria and tetanus are examples of toxoid vaccines.

e) Conjugate vaccines

These vaccines link polysaccharides (bacterial outer coating comprised of sugar molecules) to a carrier protein antigens or toxoids from the same microbe. Polysaccharide coatings disguise a bacterium’s antigens so that the immature immune systems of infants and younger children cannot recognise or respond to them. Conjugate vaccines get around this problem through the linkage of polysaccharides with a protein.

The vaccine that protects against Haemophilus influenzae type B (Hib) is a conjugate vaccine.

f) DNA vaccines

Once the genes from a microbe have been analyzed, scientists could attempt to create a DNA vaccine against it. Still in the experimental stages, these vaccines show great promise, and several types are being tested in humans. DNA vaccines take immunization to a new technological level. These vaccines dispense with both the whole organism and its parts and get right down to the essentials: the microbe’s genetic material. In particular, DNA vaccines use the genes that code for those all-important antigens.

g) 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 laboratory, a similar process can be conducted using 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. Scientist are working on both bacterial and viral-based recombinant vector vaccines for TB, HIV, rabies, and measles.


Page created on 30 January 2015

Page last updated on 27 March 2017

References: National Institute of Allergy and Infectious Diseases (NIAID)