As the world prepares for mass COVID 19 vaccination, Some of the first vaccines authorized for use such as the Moderna and the Pfizer Vaccines are mRNA Vaccines. Most Vaccines used for other infections are inactivated, live attenuated, inactivated toxins (toxoids) for bacteria or merely segments of the pathogen (subunit vaccines). mRNA vaccines are a new type of vaccine that teaches our cells how to make a harmless protein, mimicking what happens in natural infection.
The COVID 19 mRNA vaccines are given in the upper arm of the muscle. When one receives the injection, tiny lipid vesicles containing the mRNA molecules travel through the body’s fluids and merge with a special type of immune cell called antigen-presenting cells (APCs). Inside the cells, the mRNA molecules bounce around until it meets the ribosome. The ribosome produces a protein (antigen) based on the introduced mRNA sequence. The antigenic protein produced by the ribosome activates the immune system. In the case of the new COVID 19 vaccine, the spike protein will be the produced antigen.
The spike protein is presented to the immune system in 2 different pathways. In the first pathway, it is broken into fragments which are bound to another protein called MHC-I, forming a complex which leaves the endoplasmic reticulum through the secretory pathway to reach the cell surface where it is recognized by cytotoxic T cells enabling cell-mediated immunity against future infections.
In the second pathway known as the antibody-mediated immune process, spike proteins are transported to the surface of the cell after production as encoded in the introduced mRNA vaccine. In the cell surface, the spike protein is released and taken up by other antigen-presenting cells and degraded into fragments that form a complex with the MHC II proteins. The complex is transported to the cell surface where it is recognized by helper T cells. This activates antibody-producing B cells to neutralize the virus in future infections.
The advantage of mRNA vaccines is that they stimulate both types of immunity; cell-mediated and antibody-mediated immunity without using viral particles in the process. This makes mRNA vaccines considerably safer than conventional vaccines.