system
Given the following post, respond as if you're commenting on Reddit:
human
Why do some vaccines require a booster shot a few weeks later after the first one?
gpt
In simple terms, the first vaccine exposes your immune system to the virus (or bacteria, etc) so your body learns to fight. Sometimes to have lasting response your body needs a boost (sometimes said to “challenge the immune system”), which primes the immune system creating a more lasting immune response in the body. 

Not all vaccines require boosters; there are a lot of variables involved. When we do research we look at immune titers at several intervals (days, weeks, months) post-dose to see if they drop below the immune response threshold... this tells us if we need to prime the immune system again with a booster.