If WHO says there may not be immunity from being infected, doesn't that mean a vaccine won't work?

[Silent3]

16. It's an unlikely worse-case scenario that a vaccine wouldn't work at all.

There's a broad spectrum of possible outcomes, where getting the disease once and being forever immune thereafter is the best case, and having nearly no resultant immunity is the worst case.

(The reason I say "nearly" is that, if you survive the virus at all, by definition you must have at least some degree of immunity that's good enough to carry you through the time it takes to purge all of the virus already in your body from your initial infection.)

In-between cases involve short-term complete immunity which fades over time, or perhaps being better protected from the worst of the diseases symptoms if you contract the disease again.

If the disease works like one of those in-between cases, then a vaccine could still offer some protection, it just might need to be taken more frequently, and/or it might protect you from the worst of the effects of the disease without making you totally immune to it.