Not everyone who wants to be vaccinated against covid can be vaccinated.

And not everyone who is vaccinated can receive the full benefits of vaccination due to being immunocompromised.

As someone who gets a Hep B vaccine every single year, I do what I can to protect myself, but my body is incapable of producing antibodies long term, it's because of an autoimmune condition, not because the vaccine isn't widely effective. I'm always going to be more susceptible to it (and I work in healthcare so I have to keep getting the vaccine). I'm lucky I only have this issue with hep b, my other vaccines seem to have "taken", but I'm not alone, there are many people out there that have this issue with various vaccines.

My cousin had an anaphylactic reaction to the flu vaccine a few years ago, he can't take it again. He's in a vulnerable position because he has a trachy, so his airways don't have as much natural protection. He has to trust that his co-workers will stay away from him if they catch the flu, because he can't get vaccinated even though he wants and needs to.