> Unlike many infectious diseases, recovery from naturally acquired tetanus _does not usually result in immunity to tetanus_. This is due to the extreme potency of the tetanospasmin toxin. Tetanospasmin will likely be lethal before it will provoke an immune response.
That's an unusual situation that probably isn't relevant here. The way that tetanus vaccines work is that they cause the immune system to destroy the extraordinarily neurotoxic tetanospasmin toxin. It's impossible to get a natural immune response because, as the article points out, anyone with enough of it to trigger an immune response is dead. Coronaviruses triggers immunity just fine, the immunity just doesn't last and we don't really know why.
> Unlike many infectious diseases, recovery from naturally acquired tetanus _does not usually result in immunity to tetanus_. This is due to the extreme potency of the tetanospasmin toxin. Tetanospasmin will likely be lethal before it will provoke an immune response.
Quoted from https://en.wikipedia.org/wiki/Tetanus. Emphasis is mine.