The vaccine itself isn't meant to spread through the body. What it does is activate immune cells that were already hanging out in the muscle (generally macrophages), and those activated immune cells release chemical messengers called cytokines to call other types of immune cells to the site.
Jaralith t1_j51c6dh wrote
Reply to comment by Roadgoddess in Is there any difference in efficacy when a vaccine is administered somewhere other than the upper arm (e.g. on the foot)? by MercurioLeCher
The vaccine itself isn't meant to spread through the body. What it does is activate immune cells that were already hanging out in the muscle (generally macrophages), and those activated immune cells release chemical messengers called cytokines to call other types of immune cells to the site.