The mRNA vaccines don't multiply. From the moment they enter the body, their total volume can and does go down, but not up. Compare that to the virus, which clo

The mRNA vaccines don't multiply. From the moment they enter the body, their total volume can and does go down, but not up. Compare that to the virus, which clones itself and tries to evade immune defenses. Even contrasting the two vaccines is not straightforward: Moderna uses 3x the amount of mRNA as Pfizer, but then their storage requirements are also less stringent. Regarding location, it's more of an issue in the opposite direction: the fact that it is delivered through intramuscular injection gives a slower and poorer immune response in the mucosa, the first line of defense, compared to hypothetical, equivalent vaccines administered orally or nasally. Currently you have to rely on antibodies getting to the mucosa through your blood. We know that the virus can reach the brain, the GI system, etc., so there doesn't seem to be "total" protection. Of course, across different people and different immune responses, there's going to be a variable amount of viral proteins hitting throughout one's body.