How can a vaccine ever work if it's impossible to develop immunity by reacting to it?

An example with tetanus:

> 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.

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.

If there are many different strains loose in a region that don't confer immunity to each other and immunity only last a year people could be perpetually sick with the virus. A vaccine combines a variety of weak or inactive strains that give your immune system a big picture view of the virus to develop a comprehensive response to COVID-19 instead of COVID-19 Strain 93A.

The flu vaccine works the same way: scientists predict which strains are the most likely to spread and create or find weaker strains that are all combined into a single vaccine.

That's an interesting and important point. Luckily it seems to mutate 10x slower than influenza viruses and so far no different strains have been found. Finding new strains would also thwart some of the current work on vaccines being done.

https://www.theatlantic.com/health/archive/2020/05/coronavir...

I was being a bit imprecise when using the phrase "strain." In the context of virology in general, a strain is delineated by a difference in functionality - i.e. some key component of how it infects, replicated, or damages the body is different from other known cases. However, in the context of vaccines, a "strain" may function the same but might not produce some unique protein or other marker that the immune system uses to identify the pathogen, without having an effect on the rest of the virus.

COVID vaccine work is still in its infancy so we don't know yet whether the non-functional mutations we're seeing in COVID's phylogeny will effect how effective vaccines are.

There is no guarantee that a vaccine is possible.

Exactly what I'm getting at. They tend to use similar mechanisms, though vaccine being the more or less completely safe path.

What could alter the course though is if some treatment is found which makes it essentially harmless, even if immunity would be impossible.