The purpose of a vaccine is to help your immune system recognize a pathogen before it causes any harm to your body. The way vaccines achieve this is not always the same. Currently, there are four main types of vaccines [1].
Live-attenuated vaccine
This type of vaccine uses a weakened or attenuated form of the infectious organism. Attenuated means that the virus* has been modified such that it cannot be multiplied further. Live-attenuated vaccines provide a strong and long-lasting immune response. Because the virus is live, it is possible for the pathogens to go back to their original state and cause disease. As a result, these vaccines undergo rigorous testing to ensure that this will not occur. Still, live-attenuated vaccines are cost-effective and generate robust immune responses [2]. This type of vaccine was used to successfully eradicate smallpox in 1980. For more information, you can check out this post.
Inactivated vaccine
Similar to live-attenuated vaccines, inactivated vaccines employ physical or chemical processes to kill the pathogen. However, inactivated vaccines may not offer the same level and length of protection as live-attenuated vaccines, and often need additional shots to maintain immunity [3]. Nevertheless, inactivated vaccines do not present a risk of getting the disease and are more stable, which makes them easier to handle and transport.
Subunit vaccine
Instead of using the whole pathogen**, either attenuated or inactivated, subunit vaccines contain only the part of the pathogen that causes an immune response. These small parts are known as antigens***. Similar to inactivated vaccines, subunit vaccines do not always give a strong or long-lasting immune response, and repeated doses are commonly needed. Subunit vaccines are divided into three different categories: protein-based subunit vaccines, polysaccharide vaccines, and conjugate subunit vaccines. The first group, protein-based subunit vaccines, utilizes a specific protein**** isolated from the pathogen. One of the disadvantages is that the isolated proteins expressed outside the context of the virus may achieve a different shape compared to the same protein in the live virus. This may affect the immune response by interrupting the antibodies-protein interactions [4].
Antibodies are elements of the immune system that recognizes foreign substances, like viruses and bacteria, and neutralizes them [5]. Antibodies interact with proteins through a lock-and-key-type mechanism where the shape of the protein is very important. Therefore, a misshaped isolated protein will encourage the production of antibodies that are not optimized to attack the actual virus. Extensive testing is performed to ensure an effective immune response and proper protein structure for antibody production.
On the other hand, some bacteria have a protective capsule surrounding the cell that enables infection. This capsule is composed of sugars (polysaccharides). Polysaccharide vaccines (another type of subunit vaccines) cause an immune response against specific molecules on the surface of the protective capsule. However, this type of vaccine does not provide long-lasting protection, and it is not very effective for babies or young children. To enhance the immune response against the polysaccharides present on the protective capsule, researchers developed conjugated vaccines. These vaccines connect the polysaccharide to another compound that induces a strong, long-term immune response, like diphtheria or a tetanus toxoid protein [6].
Toxoid Vaccines
Prevent infections with minimum risk. Toxins are poisonous proteins released by some bacteria [7]. The toxoids, which are inactivated toxins, are recognized by the immune system and induce a strong immune response. The main advantages of toxoid vaccines are stability and safety. They cannot become virulent or cause the disease they prevent.
Interestingly, there is an emerging type of vaccine that involves the use of nucleic acids***** (genetic material) to give instructions to one’s cells on how to produce the antibodies needed to cause an immune response. You can read more about this type of vaccine here.
*Virus - it can be considered a very small and basic life form or an aggregate of molecules that is simpler than a cell. They can be found in the environment or inside living organisms. It has to infect a cell and use its machinery to multiply, since it cannot multiply by itself. Viruses consist of nucleic acids (DNA or RNA), a protein capsid that contains the nucleic acids and, sometimes, an external envelope of lipids. The flu, measles, AIDS and Covid-19, for instance, are caused by viruses.
**Pathogen - any microorganism that can cause infection and disease.
***Antigen - a molecule or substance that is considered foreign to the organism. It can be a toxin, a small part of a microorganism in cases of infection or it can even be a small part of our own body in cases of autoimmune diseases. The antigens stimulate an immune response and the production of antibodies.
****Protein - a molecule that forms the structure and gives function to organisms at the most basic level. Proteins consist of amino acids, which can be combined in different sequences to form different protein structures with different functions in cells.
*****Nucleic acids - a type of molecule that contains genetic information in cells (see genetic code). The two types of nucleic acids are DNA and RNA.
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Written by: Alba
Edited by: Adrian and Natasha
BioDecoded is a volunteer group committed to sharing accurate scientific information. We cannot offer any specific health advice. If you have any doubts about getting vaccinated due to previous health conditions, please speak with your healthcare professional or family physician. Your doctor can revise your medical history and advise you on the best path to follow. If you have any questions about this topic, please comment or send them to our email.
References:
How Drugs are Reviewed in Canada – Canada.ca. 2015 Available at: https://www.canada.ca/en/health-canada/services/drugs-health-products/drug-products/fact-sheets/drugs-reviewed-canada.html. (Accessed: 13th April 2021)
Minor, P. D. Live attenuated vaccines: Historical successes and current challenges. Virology 479–480, 379–92 (2015). Available at: https://pubmed.ncbi.nlm.nih.gov/25864107/
Delrue, I., Verzele, D., Madder, A. & Nauwynck, H. J. Inactivated virus vaccines from chemistry to prophylaxis: merits, risks and challenges. Expert Rev. Vaccines 11, 695–719 (2012). Available at: https://pubmed.ncbi.nlm.nih.gov/22873127/
World Health Organization. Welcome – WHO Vaccine Safety Basics. Available at: https://vaccine-safety-training.org/home.html. (Accessed: 13th April 2021)
Fritzell, B. [Conjugated vaccines]. Therapie 60, 249–55 (2005). Available at: https://pubmed.ncbi.nlm.nih.gov/16128267/
Shakya, M. et al. Phase 3 Efficacy Analysis of a Typhoid Conjugate Vaccine Trial in Nepal. N. Engl. J. Med. 381, 2209–2218 (2019). Available at: https://www.nejm.org/doi/full/10.1056/NEJMoa1905047
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