Here’s the first thing that came to my mind when I heard of the mRNA COV vaccine.
mRNA encoding the information of the coronavirus spike protein gets into cells.
Host cells that have taken up the mRNA make spike proteins in the cytoplasm.
Peptidic parts of the spike protein get displayed on MHC1 on cell surfaces of all cells that have taken up the mRNA. (Recall, most nucleated cells have MHC1.)
Cytotoxic T cells would recognize these foreign antigens on MHC1 and naturally kill all host cells that display them.
In other words, all cells that have taken up the mRNA may die. What if some of these mRNA molecules travel through the circulatory system and get into more precious cells… like egg cells, or neural stem cells?
Although seemingly different from mRNA vaccines, some COV vaccines that use adenovirus vectors to enter host cells and force self-cells to make spike proteins will also have the same problem.
Vaccines that work mostly via the MHC2-antigen complex using extracellular antigens are fundamentally safer and different from vaccines that work via infection (or penetration into self-cells, whether it be through adenovirus or mRNA with the help of liposome-like materials) that would naturally expose the infected cells to the attacks by cytotoxic T-cells or NK cells (in the event where viral infection leads to the decreased MHC1).
It seems like while everyone focuses on the antibody production by the B cells, the presence of T cells and the danger that they may bring is forgotten.
This is an example that shows that safety and efficacy should always be considered together, from the very beginning. The human body is a very complex system. A brute-force, simple method often leads to unintended parts of the body being severely affected.
Side effects of drugs that focus only on small (or no) parts of the immune system can often be projected on a mechanistic level even before they are made.