There are many diseases that, once experienced, won’t be experienced again, such as measles or chicken pox. The immune system gears up to eliminate them. B cells recognize viruses and produce antibodies for it. However, there are only a few of these cells for each antibody.
When a particular disease is recognized by specific B cells, the B cells turn into plasma cells, clone themselves, and start pumping out antibodies. This process takes time, but the disease runs its course and is eventually eliminated. However, while it is being eliminated, other B cells for the disease clone themselves but do not generate antibodies. This second set of B cells remains in the body for years, so if the disease reappears it is able to eliminate it immediately before it can become a major problem in the body.
A vaccine is a weakened form of a disease. It is either a killed form of the disease, or it is a similar, but less virulent strain. Most recently, now that codes have been identified and understood. it can also be genetically created. Once injected, the immune system mounts the same defense, but because the disease is different or weaker, no symptoms of the disease appear. Vaccines exist for many diseases, both viral and bacterial. Examples are: measles, mumps, whooping cough, tuberculosis, smallpox, polio, typhoid, etc.
Vaccines are ineffective for colds and some influenzas, as well as other diseases. This is usually because there is such a rapid mutation rate or different strains present that it is impossible to inject all. Vaccine development cannot begin until a new virus has emerged and the vaccine strain is available. At this point, the Department of Health and Human Services (DHHS) is working closely with vaccine manufacturers to expand the annual influenza vaccine capacity.
There are basically five types of vaccines.
Sub-unit vaccines are related to the recombinants (virus, bacterium, or other organism in which the genetic material has been articificially modified) and only utilize a protein fragment. They contain purified antigens rather than whole organisms; an example is the Bordetella pertussis antigens included in the acellular DPT vaccine. They are among the newest type, and are completely safe, except for rare adverse reactions. Unfortunately, they also tend to be the least effective. None are currently in use.
These vaccines are produced by exposure to denaturing agent that results in loss of infectivity without loss of antigenicity. Little or no risk if properly inactivated. These are advantages and disadvantages of inactivated vaccines.
Polysaccharide vaccines are a unique type of inactivated subunit vaccine composed of long chains of sugar molecules that make up the surface capsule of certain bacteria. Pure polysaccharide vaccines are available for three diseases:
The immune response to a pure polysaccharide vaccine is typically T-cell independent, which means that these vaccines are able to stimulate B-cells without the assistance of T-helper cells. T-cell independent antigens, including polysaccharide vaccines, are not consistently immunogenic in children <2 years of age. Young children do not respond consistently to polysaccharide antigens, probably because of an immature immune system. Repeated doses of most inactivated protein vaccines cause the antibody titer to go progressively higher, or “boost.”
Repeat doses of polysaccharide vaccines do not cause a booster response. This is not seen with polysaccharide antigens. Antibody induced with polysaccharide vaccines has less functional activity than that induced by protein antigens. This is because the predominant antibody produced in response to most polysaccharide vaccines is IgM, and little IgG is produced. In the late 1980s, it was discovered that the problems noted above could be overcome through a process called conjugation.
Conjugation changes the immune response from T-cell independent to T-cell dependent, leading to increased immunogenicity in infants and antibody booster response to multiple doses of vaccine. The first conjugated polysaccharide vaccine was for Haemophilus influenzae type b (Hib). A conjugate vaccine for pneumococcal disease was licensed in 2000.
The use of virus with reduced pathogenicity to provide immune response without disease is done with live viruses. They may be a naturally occurring virus (e.g. Jenner, cowpox), or artificially attenuated (oral poliovirus vaccine -OPV).
Viruses were formerly grown in animal cells, but human diploid cells are now commonly used. These are screened for all known viruses. However biological additives are required for their growth, e.g. foetal calf serum. There is always the risk therefore of contamination with a biological agent. Cultures are seeded from safe virus stocks.
While theoretically promising, anti-staphylococcal vaccines have shown limited efficacy, because of immunological variation between Staphylococcus species, and the limited duration of effectiveness of the antibodies produced. Development and testing of more effective vaccines is under way.
Live typhoid vaccine (Ty21a) is Salmonella typhi bacteria that has been genetically modified to not cause illness. Live attenuated influenza vaccine (LAIV) has been engineered to replicate effectively in the mucosa of the nasopharynx but not in the lungs.
Vaccine antigens may also be produced by genetic engineering technology. (See discussion later on genetic engineering). These products are sometimes referred to as recombinant vaccines. An example of a genetically engineered vaccine currently available in the United States is the vaccine for Hepatitis B.
Vaccines are products that have been developed to induce or mimic different diseases so that the human body will develop immunity and will not contract that infection if exposed. While there is not a vaccine for every infectious disease, and there is some discussion about the risk/benefit of various childhood immunizations, the development of vaccines continues to be a high priority in the fight against disease.