Badaboom TISH!

It's not at all unusual to exhibit symptoms of sickness after a vaccine. As Trev has correctly pointed out, it is because it's the body's immune system learning how to fight the inactive form of the pathogen that's just been injected into the body. If you think of your white blood cells as soldiers then the sick feeling is because they're undoing grueling combat simulation training.

If you ever get the chance, there's a really educational anime series called "Cells At Work." It explains how the body works at a cellular level in a pretty fun and entertaining way, although not suitable for little kids due to the violence and gore scene during an immune response (although IRL it's actually far more violent and gory!). I was introduced to this series by a friend of mine who's a biology teacher and I know a few other bio teachers who encourage their senior students to watch this show if they're struggling to understand concepts of microbiology.

Some of my favourite parts of the show includes:
* Sneezes are shown as being rockets launching.
* Red blood cells all carry boxes labelled O2 or CO2
* All cell types have different uniforms/clothes; white blood cells, Killer T Cells, Helper T Cells, Macrophages, B Cells, Dendrites etc.
* Memory Cells carry have books that document past infections, instructing other cells on how to fight new infections. <---this is where vaccines come into play because while the immune cells are fighting it's these Memory Cells that are "remembering" how to fight.
* All of the body's cells look like people whereas pathogens look like monsters. Infected cells look like zombies.
* When the body is infected everyone feels hot. The body has a fever!
* The stomach is portrayed as a food assembly line, and when the body is infected an alarm is sounded with the entire assembly line being forcibly shut down (and all the worker cells going, "Aawww!") -- loss of appetite!
etc etc.

It's a shame that there isn't a more kid friendly version without all the blood and gore, because it's otherwise quite a fun way to learn about the body. The show has even been praised by doctors!