B-Cell Vaccines as a general preventative measure

First, a bit of background: a relative recently unfortunately was diagnosed with Stage 4 gastric cancer. I enquired as to whether the cancer was HER-2 positive but the response was that as the cancer was so "diffuse", it was not possible to get enough of a sample to know whether it was or not.

Which made me think that, as gastric cancer in particular is often not picked up until quite late - and even then it may not be possible to get enough of a sample as to diagnose if it was HER-2 positive - why not give the vaccine (once approved) to people as a general preventative measure, rather than as a treatment? Particularly those at higher risk of gastric cancer. There seem to be no safety concerns and I understand it shouldn't be too expensive, so on the plus side it may prevent cancers that are newly forming before they would otherwise be detected.

Not sure if this has been discussed before, keen to know your thoughts.