It doesn't take very long for the mRNA to break down, a few hours. It's perhaps overly simplified, but the first shot teaches the immune system what to look fo

It doesn't take very long for the mRNA to break down, a few hours. It's perhaps overly simplified, but the first shot teaches the immune system what to look for and the second shot causes it to prepare a bunch of antibodies that will attack the virus (in some sense, the second shot is a reminder to keep defending against the virus, and the immune system does that by making antibodies).