system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why does my arm feel sore after getting a shot at the doctors?

Just got a shot, arm feels really sore if I move it. Explanation?
gpt
The trauma of the needle itself isn't primary reason you're sore.  You are sore because the vaccine is composed of adjuvants in addition to the inactive portions of virus that stimulate the immune system (so as to create better immunity) but this also creates local inflammation in the process.