Also, it's hard to comprehend how much the virus changes within one infected person. It's a common talking point in HIV seminars: the diversity of HIV strains in one infected individual exceeds that of flu worldwide in any given year [1]. Additionally, the parts of the virus that are more or less universally conserved (and thus are good targets for a vaccine) are hidden except for the moments where the virus binds the target cell. If I wanted to fabricate a difficult to vaccine pathogen, it would be very similar to HIV-1.

1. https://pubmed.ncbi.nlm.nih.gov/11714622/