system
Given the following post, respond as if you're commenting on Reddit:
human
What are the advantages of mRNA / lipid droplet vaccines over viral vector and other existing vaccine types?

In my simplistic model, both the lipid and viral vectors achieve the same thing: deliver rna/DNA to the cell so it can produce virus proteins to train the immune system. But the lipid vaccines are difficult to store and administer. (Perhaps also hard to manufacture?) Anyways, I’m guessing there’s an advantage to make the trade off worth it. I’m curious what that is!
gpt
People can have pre-existing anti-vector immunity to viruses such as adenovirus, and even when there is no pre-existing immunity (like with the chimpanzee adenovirus used by Oxford/AZ) you can still end up developing immunity to the vector after the first dose. 

That immunity to the viral vector can decrease the level of expression of your desired protein and is thus counterproductive for a vaccine. 

The mRNA vaccines avoid that issue, so that's one advantage.