As another note, the mRNA platform has a few concerns that the viral vector vaccines do not. In particular, the delivery of the mRNA payload is done via lipid

As another note, the mRNA platform has a few concerns that the viral vector vaccines do not. In particular, the delivery of the mRNA payload is done via lipid nanoparticles that are not in any way targeted to a specific cell type. The European Medicines Agency released a report on the distribution of the mRNA found in various organs: https://www.ema.europa.eu/en/documents/assessment-report/spikevax-previously-covid-19-vaccine-moderna-epar-public-assessment-report_en.pdf https://www.ema.europa.eu/en/documents/assessment-report/spi... > "Besides injection site [muscle] and lymph nodes [proximal and distal], increased mRNA concentrations (compared to plasma levels) were found in the spleen and eye. Both tissues were examined in the frame of the toxicological studies conducted with mRNA-1273 final vaccine formulation. Low levels of mRNA could be detected in all examined tissues except the kidney. This included heart, lung, testis and also brain tissues, indicating that the mRNA/LNP platform crossed the blood/brain barrier A similar 2017 study was conducted using mRNA coded for luciferase as a tracking mechanism: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5475249/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5475249/ And it, likewise, found the protein was produced in the muscles, lymph nodes, spleen, liver, heart, bone marrow, kidney, lung, stomach, rectum, intestines, testes, and brain. One of the main draws of using the lipid nanoparticle as a delivery mechanism for other payloads in the past has specifically been that the lipid nanoparticle can readily cross the brain-blood barrier (BBB). And the EMA paper/quotation referenced above seems to verify that this is, in fact, the case. A question that both platforms have to contend with is: "what, if any, risks are associated with having your cells producing and expressing a pathogenic antigen that your immune system is to identify and attack?" But one that the lipid nanoparticle delivery system has to contend with is the same question, but in the context that it has very little differentiation WRT where it is going to be deployed and expressed. With the viral vector vaccines, the delivery mechanism is a living virus, and has much more predictable infection pathways. For one, our bodies have millions of years of evolution for dealing with viral infection, and particularly safeguarding the parts of our body least desirable for infection. Viruses have a very hard time crossing the blood brain barrier, whereas lipid nanoparticles do not. There are, meanwhile, concerns associated with the viral vector vaccines which are not associated with the mRNA vaccines, but I've yet to see this sufficiently addressed as a major shortcoming of the mRNA platform. Targeted delivery is a very important topic in the world of gene therapies, and seems to have been completely neglected in the available platforms.