Perhaps the part that confuses you. In order to maximize immune response, you want the antigen to be presented for a longer time so that it will be properly recognized by the immune system. This is what has been referred to as depot effect in this thread. As I mentioned, the original assumption was that releasing it directly in the bloodstream would lead to relatively fast clearance, thus minimizing the response. The only way to increase the response was to increase concentration, which can be risky. As such vaccines were developed to be released more slowly via intramuscular injection, with the added benefit of minimizing discomfort and adverse effects (as already mentioned). The development of adjuvants has provided alternatives. The adjuvants can now provide the desired depot effect. However, since there are no obvious benefits in IV delivery under normal circumstances, it is generally not done. So to re-iterate, using adjuvants that provide depot effects IV delivery is feasible. But it is not advantageous, has higher risk of issues and requires additional protocol development. So it is generally not done for routine vaccination.