Types of Vaccines
Vaccines are the single greatest advance in the history of public health interventions. Their efficacy and safety have been demonstrated again and again, and as the COVID-19 pandemic drags on, the latest prospect of a vaccine offers a glimmer of hope that the end of the pandemic could be approaching—especially for those working on the frontlines. In this figure, we explore the various types of vaccines we currently have in use and their immunologic effect.
Starting with the killed/inactivated vaccines: these vaccines are very effective and easy to produce. Additionally, the vaccine contains all the major components with which the body would react if an infection were to occur. The challenges with this type of vaccine include (1) may not be as immunogenic, (2) does not protect against simple mucosal infections, and (3) if the bacterium produces a toxin, the toxin could still be present—not used in these cases.
Next, we have the toxoid vaccine. This is a very safe vaccine because it takes the part of the bacterium that causes disease (the toxin), alters the structure so that it does not have an effect on the body, and allows the body to mount an immune response to the toxin. This type of vaccine is great for toxin producing bacteria, but some challenges include (1) does not prevent the colonization of the bacterium (not usually an issue because colonization does not equal infection and colonization without toxin activity could be completely benign), (2) the vaccine does not mount a mucosal immune response with sIgA (also not usually an issue since the toxin usually has systemic effects; however, for localized toxin effects on a mucosal membrane like ETEC—see prior micromeded post—another form of vaccination is needed).
The next type of vaccine in the figure is the conjugate vaccine. Many other vaccines are clumped into the category and the prior category including carbohydrate, subunit, or recombinant. For the conjugate vaccine, the antigen of interest is not strongly immunogenic—a carbohydrate capsule usually mounts a weak, T-cell independent immune response which leads to the production of the less-specific, less immunogenic IgM antibody. Additionally, kids (the target population for many of these vaccine) do not mount a T-cell independent response, so simply injecting the carbohydrate into the arm would not work. So, to get around this, scientists conjugated the carbohydrate target to a protein target from a different pathogen, so the body mounts a T-cell depended, IgG response. This is a very clever way of tricking the immune system into making highly active antibodies against multiple pathogens at the same time. The weakness, like the others, is that it does not mount a mucosal or cell-mediated immune response.
The next vaccine type is the nucleic acid vaccine. This type of vaccine has only recently been approved for use in humans, but the study of this vaccine type has been around for decades. The idea behind this vaccine is allow cells to take up a portion of RNA from the virus that produces the antigenic portion of the virus (in the case of SARS-CoV-2, this antigen is the spike protein). Once the RNA is in the human cell, the ribosomes will transcribe the message and make the protein. Some of the protein will then be expressed on MHC-I (the “self” receptors) as well as on the cell surface or secreted (depending on the protein). The MHC-I receptor expression will mount a CD8+ T-cell response, and the expression of the protein itself can be processed and expressed on MHC-II receptors (the non-self receptor) on dendritic cells and other antigen presenting cells which allows the body to mount a B-cell response with the help of the T-helper cell. In the end, this vaccine mounts a strong IgG/IgM response as well as a cell-mediated immunity. The cell mediated immunity is amazing since that is how infected cells are recognized and killed during a viral infection. The weakness is that that this type of vaccine does not mount a mucosal immune response.
Finally, we explore the live-attenuated vaccine. This is the gold standard of vaccine immunology because it can be administered in the same way the virus enters (through the GI tract as a liquid droplet or pill or through the respiratory tract as a nasal mist) leading to a robust cell-mediated immunity, systemic IgG response, AND mucosal immunity with the secretion of IgA. The secretion of IgA is amazing because this antibody can effectively neutralize the pathogen before it even attaches to the mucosal membranes. There have been some challenges with this vaccine despite its overall safety which includes the possibility of reactivation and some GI symptoms in those delivered via that route. Of course, immunosuppressed individuals are not recommended to take these vaccines fore the aforementioned reasons.
For more info on the COVID-19 vaccine, visit this website.
For CDC-recommended immunization schedules, visit this website.
To see which vaccines belong to which category, check out this website
For a great website detailing the immunologic responses behind each of these vaccine classes, visit this website.
Figure created with Biorender.com
The idea of this figure was submitted by @azmagier via the #micromeded suggestions form
