What exactly does a vaccine shot do?

I’ve found a vaccine shot in a pharmacy, it says that it provides long lasting protection against diseases.
Does it protect against ALL diseases? Or just the ambient ones? Or the zombie disease? Is it permanent or does it just last for a certain time?

The vacc shot is basically a super-vitamin. It gives a substantial increase to your (hidden) Health stat, meaning that you’re pretty much set against getting a Cold or Flu (the ambient diseases) for a while. Since your Health trends to 0 (middling), it’ll wear off in time.

The infection you get from zed bites is a mundane infected-wound. Everyone’s got the reanimate-on-death disease already.