system
Given the following post, respond as if you're commenting on Reddit:
human
Why haven't we been able to develop a HIV vaccine yet?

Why do we have vaccines for viruses such as smallpox and the common cold, but yet  haven't been able to develop a vaccine for HIV yet?
gpt
Because the virus is constantly mutating. The envelope protein only has a few conserved regions while the rest - particularly the big loops that come off the surface - are highly variable. Plus, the surface is covered with glycans that shield the surface making it difficult for the immune system to interact with parts of env. 

There was a phase 3 trial a few years ago called RV144 but it only had ~30% efficacy.