system
Given the following post, respond as if you're commenting on Reddit:
human
So, I just got some vaccines and now I’m thinking about them. It seems like vaccines would be more effective if administered intravenously, so why are many vaccines given via the intramuscular route instead?
gpt
There are multiple arguments for that. The reasons are historical, practical and scientific.

1/ Traditionally, vaccines were injected subcutaneously and we thought that the buttocks were an appropriate site for vaccination. Now, we are injecting other muscles like the deltoid.

2/ Some vaccines are meant to be injected to the whole population. It is much easier to perform an intramuscular or a subcutaneous injection, than an intravenous one.

3/ Intramuscular injections are effective as the muscle is highly vascularised.

4/ Intramuscular injections don't scarify the skin, don't cause abscesses and don't cause granulomas, like some subcutaneous injections.

5/ Intramuscular injections, contrary to intravenous injections sustain a "depot effect", during which the antigen is released slowly and progressively into the blood circulation, ensuring a continuing stimulus on the immune system.

6/ Intravenous injection of adjuvants contained in the vaccine could have a higher toxicity than if injected intramuscularly (LPS, aluminium).