Classification of Vaccines
The effectiveness of the vaccine is important for every vaccine that is put into practice. The probability of protection from the disease for which the vaccine is administered indicates the effectiveness of the vaccine and is determined by the level of attack rates between those who are vaccinated and those who are not. While the problem of protection is mentioned for vaccines with an efficacy of less than 80.0%, it can be said that vaccines with an efficacy of 90.0% and above are sufficiently protective, while vaccines in between have a level of protection that will not cause a problem. The effectiveness of the vaccine is monitored not only by clinical studies but also by field effectiveness determined as a result of surveillance studies. Field effectiveness can also be affected by many implementing factors other than the vaccine, such as the way the vaccine is administered, compliance with the cold chain rules.
There are many vaccines developed or under development against various infectious diseases. These vaccines are classified in different ways according to their various characteristics.
Classification of vaccines by production method
Live, Attenuated Vaccines:
Using a method of attenuation of “wild” viruses or bacteria (such as yellow fever, poliovirus, measles) using a similar causative type (such as Edward Jenner’s use of the “coxpox” type in flowers) or multiple passages in different reproductive media (cell cultures or animal embryos) is obtained. A small enough dose to initiate an immune response is administered, where the virus or bacteria is multiplied. The disease caused by the wild form does not occur, or symptoms that are much milder than the natural disease, which can be defined as a side effect, occur. The resulting immune response is as strong as that obtained by natural disease. In general, an adequate response is obtained with a single dose of vaccine, but sometimes two doses may be required. In oral applications, a single dose is not sufficient. Live vaccines should not be used in people with weakened immune systems (those with immunodeficiency, those using immunosuppressive drugs, pregnant women) as they may cause serious infections.
Antibodies transmitted from the mother through the placenta or antibodies received through blood transfusion may reduce the immune response to live vaccines. Therefore, timing of vaccination is important.
Live vaccines are extremely sensitive to heat and light.
MMR, Chickenpox, BCG, Influenza, Rotavirus, Typhoid, Yellow fever vaccines are in this group.
Dead, Inactivated Vaccines:
It is obtained by inactivation of all or part of the agent by chemical, heat or radiation (polio (IPV, Salk), cholera, hepatitis A, plague, rabies). It is easy to use, transport and obtain. They are vaccines that are not live, that is, do not multiply, and therefore can be used in people with weakened immune systems. They are not affected by antibodies transmitted from the mother through the placenta or by antibodies received through blood transfusion. Multiple doses are required to maintain the immune response. The first dose prepares the immune system, the immune response develops after the second or third dose. As antibody levels decrease over time, a booster dose is required. Adjuvant may be added to induce adequate immune response. However, if the agent is not fully inactivated during vaccine production, infection may occur. On the other hand, the risk of side effects such as allergic reactions increases as high doses of antigens are required.
Inactivated vaccines can be divided into four groups:
- Whole-cell inactivated vaccines: Contains all of the viruses and bacteria produced in the culture medium and then killed by a physical or chemical process (Inactivated Polio, hepatitis A, Pertussis, rabies vaccines)
- Subunit vaccines: A part of the relevant virus or bacteria required for the immune response is used. In these vaccines, antigen protein, polysaccharide or their combination (Hepatitis B, acellular pertussis vaccine)
- Subunit / Conjugated Polysaccharide Vaccines: A polysaccharide is added to the superficial protein of the bacteria using chemical methods. With this process, a long and permanent immune response is created. The immune response is mediated by B lymphocytes without the help of T lymphocytes. Therefore, in children younger than 2 years of age who do not have adequate immune development, an effective response does not occur with these vaccines.
In the conjugated polysaccharide type, carrier proteins are used to increase their immunity. Purified capsule polysaccharide antigens are conjugated with carrier proteins to increase their immunity-generating properties (diphtheria toxin CRM 197 (non-toxic mutant), Tetanus toxoid).
- Subunit/Conjugated Recombinant vaccine: Genes encoding an immunogenic protein are isolated, recombined into a vector (bacterial, mammalian or fungal). The recombinant antigen begins to be expressed in this vector. It is purified biochemically as carbohydrate or protein from the cells produced (hepatitis B vaccine, Haemophilus influenza type b (Hib), pertussis (DTaP), pneumococcus, meningococcus, HPV). The vector used is both safe and easy to produce, the vaccine can be easily stored, and its production is economical. However, the development phase is difficult and expensive.
- Toxoids: Produced by inactivating toxins produced by bacteria by heat, chemicals, or other methods. Adjuvant is used to prolong absorption and increase antigenic effect (Diphtheria, tetanus toxoids).
- Recombinate vaccines: Produced by Recombinant DNA technology, which uses a combination of DNA obtained from two or more sources. A surface antigen, capsid protein or hemagglutinin is formed as a result of transferring a part of the viral gene to the yeast cell or virus gene (Hepatitis B, HPV and some influenza vaccines).
- Vaccines Containing mRNA and DNA: When a cell that has received the RNA packages dies, it leaves behind many spike proteins (spikes) and residues containing protein fragments. When spike protein fragments on the surface of the cell are detected by T lymphocytes, B lymphocytes are stimulated and a few of them are locked into spike proteins. Antibodies that form and begin to multiply can attach to the spike proteins of the virus, mark them for destruction and prevent them from sticking to other cells, preventing infection. mRNA vaccines do not cause infection. However, it must be stored and transported at low temperatures (-70°C) in order to remain the same.
With this method, which is a new technology, vaccines can be developed for some diseases such as Rabies, HIV, Zikavirus and cancer (Covid-19 Vaccine – Pfizer Biontech and Moderna vaccines).
4.Vector Vaccines: These are vaccines created by adding the genetic information of the antibody-forming antigenic structure of the targeted microorganism to the modified viruses (Covid-19 Vaccine – SputnikV, ZEBOV, AZD1222).
Classification of vaccines by form of administration
Vaccines can be classified as routine vaccines, vaccines to be administered when needed, and optional vaccines according to the method of administration of the vaccine. Apart from this classification, vaccines can be mentioned according to the method of administration ( oral or parenteral).
- Routine vaccines: These are the vaccines that are planned to be administered to the whole population within a certain plan. The number of vaccines in this group changes over time.
- Vaccines to be applied when necessary: These are the vaccines that are intended to be administered to certain risk groups, not to the whole population.
- Optional vaccines: These are vaccines that are applied on demand of individuals.