Live Attenuated Vaccines
For these types of vaccines, a weaker, asymptomatic form of the virus or bacteria is introduced into the body. Because it is weakened, the pathogen will not spread and cause sickness, but the immune system will still learn to recognize its antigens and know to fight in the future.
For most of the modern vaccines this “weakening” is achieved through genetic modification of the pathogen either as a naturally occurring phenomenon or as a modification specifically introduced by scientists.
Because these vaccines are so similar to the natural infection that they help prevent, they create a strong and long-lasting immune response. Even 1 or 2 doses of most live vaccines can give us a lifetime of protection against a germ and the disease it causes.
However, there are some limitations of these vaccines that is since they contain living pathogens, these vaccines are not given to people with weakened immune systems, such as people undergoing chemotherapy or HIV treatment, as there is a risk the pathogen could get stronger and cause sickness.
Additionally, these vaccines must be refrigerated at all times so the weakened pathogen doesn’t die. So these vaccines can’t be used in countries with limited access to refrigerators. Some examples of live-attenuated vaccines are Measles, Mumps, Varicella (chickenpox), Influenza (nasal spray) and Rotavirus.
Inactivated Vaccines
For these vaccines, the specific virus or bacteria is killed with heat or chemicals, and its dead cells are introduced into the body. Even though the pathogen is dead, the immune system can still learn from its antigens how to fight live versions of it in the future. These vaccines can be freeze dried and easily stored because there is no risk of killing the pathogen as there is with live attenuated vaccines.
They are also safer, without the risk of the virus or bacteria mutating back into its disease-causing form. However, inactivated vaccines do not always create such a strong or long-lasting immune response as live attenuated vaccines. Some example of inactivated Vaccines are Polio (IPV), Hepatitis A and Rabies.
RNA vaccines
RNA vaccines use mRNA (messenger RNA) inside a fat membrane. This fatty cover both protects the mRNA when it first enters the body, and also helps it to get inside cells by fusing with the cell membrane. Once the mRNA is inside the cell, machinery inside the cell translates it into the antigen protein.
The mRNA typically lasts a few days, but within that time sufficient antigen is made to stimulate an immune response. It is then naturally broken down and removed by the body. Some example of RNA vaccines are – Pfizer BioNTech and the Moderna COVID-19 vaccines.
DNA Vaccines
Such vaccines are still in experimental stages. If successful developed, they would dispense with all unnecessary parts of a bacterium or virus and instead contain just an injection of a few parts of the pathogen’s DNA. These DNA strands would instruct the immune system to produce antigens for fighting the pathogen all by itself. As a result, these vaccines would be very efficient immune system trainers.
DNA vaccines are typically administered along with a technique called electroporation. This uses low level electronic waves to allow the bodies’ cells to take up the DNA vaccine. They are also cheap and easy to produce and more stable than mRNA so doesn’t require the same initial protection. DNA vaccines for influenza and herpes are currently in human testing phases.
Viral vector vaccines
Viral vector vaccines use a modified version of a different virus as a vector to deliver protection. Viral vectored vaccines use harmless viruses to deliver the genetic code of target vaccine antigens to cells of the body, so that they can produce protein antigens to stimulate an immune response. Viral vectored vaccines are grown in cell lines and can be developed quickly and easily on a large scale.
Viral vectored vaccines are very cheap to produce compared to nucleic acid vaccines and many subunit vaccines. Several different viruses have been used as vectors, including vesicular stomatitis virus (VSV), measles virus, and adenovirus, which causes the common cold. . Viral vector vaccines are also used to protect against COVID-19.
Subunit Vaccines
For some diseases, scientists are able to isolate a specific protein or carbohydrate from the pathogen that, when injected into the body, can train the immune system to react without provoking sickness.
Using this technique subunit vaccines are produced and that is why subunit vaccines do not contain any whole bacteria or viruses at all. Instead, these vaccines typically contain one or more specific antigens from the surface of the pathogen.
The major advantage of subunit vaccines over whole pathogen vaccines is that the immune response can focus on recognition a small number of antigen targets. Also with these vaccines, the chance of an adverse reaction in the patient is very low, because only a part of the original pathogen is injected into the body instead of the whole thing.
Again there are some limitation of these vaccines which is identifying the best antigens in the pathogen for training the immune system and then separating them is not always possible. Only certain vaccines can be produced in this way like Influenza, Haemophilus Influenzae Type B (Hib), Pertussis (part of DTaP combined immunization), Pneumococcal and Meningococcal.
Let us have a look at some types of subunit vaccines.
Toxoid Vaccines
Some bacterial diseases damage the body by secreting harmful chemicals or toxins. The immune system recognises these toxins in the same way that it recognises other antigens on the surface of the bacteria, and is able to mount an immune response to them.
For these bacteria, scientists are able to deactivate some of the toxins using a mixture of formaldehyde and water. These dead toxins are then safely injected into the body. Remember that they are called ‘toxoids’ because they look like toxins but are not poisonous.
The immune system learns well enough from the dead toxins to fight off living toxins if they ever make an appearance again. Some common example of toxoid vaccines are Diphtheria and Tetanus.
Conjugate Vaccines
With some bacteria, to get protection from a vaccine we need to train the immune system to respond to complex sugars on the surface of bacteria rather than proteins. It was found that they did not work well in babies and young children. So to solve this problem, scientists link an antigen from another recognizable pathogen to the sugary coating of the camouflaged bacteria.
In most conjugate vaccines, the sugar coating is attached to diphtheria or tetanus toxoid protein. As a result, the body’s immune system learns to recognize the sugary camouflage itself as harmful and immediately attacks it and its carrier if it enters the body. Example of conjugate vaccine is Haemophilus Influenzae Type B (Hib).
Outer Membrane Vesicles (OMV) Vaccines
Outer Membrane Vesicles (OMVs) are naturally produced by bacteria and are like bubble of the bacterial outer cell membrane, which contains many of the antigens found on the cell membrane.
These vaccines can can also be modified so that toxic antigens are removed and antigens suitable for stimulating an immune response can be kept. Example of OMV vaccine is MenB vaccine (meningococcal B vaccine)
Recombinant Vector Vaccines
These experimental vaccines are similar to DNA vaccines in that they introduce DNA from a harmful pathogen into the body, triggering the immune system to produce antigens and train itself to identify and combat the disease. The difference is that these vaccines use an attenuated, or weakened, virus or bacterium as a ride, or vector, for the DNA.
Basically what actually happen is that scientists take a harmless pathogen, dress it in the DNA of a more dangerous disease, and train the body to recognize and fight both effectively. Recombinant vector vaccines for HIV, rabies, and measles are currently being developed.
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