>2. So of what use is another tool? (Why develop a grammar checker?)
This isn't quite the right analogy, because a vaccine doesn't do something different from PrEP, it just does it in a different way. At the individual level it might make a compelling case (but more of that in a bit), but for the systemic purpose of eradicating the disease, not so much; not until it's given out during childhood like other vaccines.
>But there is also this: the inconvenient drugs that block transmission are mainly used by HIV positive people and people who are exposed to them. Someone not in that situation is not going to get on a train of injections every three months. (I've never even heard of these drugs until this submission and its discussion threads.) Whereas a HIV vaccine could be something more interesting to the general population. There will likely be some people who get the vaccine who ignored the transmission-blocking drugs.
The fact of the matter is, HIV is a shitty virus, by which I mean it's an inefficacious one. It has a bad spreading mechanism and it has effects that are too severe and too obvious. The reason people ignore those drugs (besides them being expensive) is that HIV is so rare, and there are overall better methods to avoid catching it. So most people will likely never have sex with a carrier, and of those who would, most will already be using condoms. Among this latter group, I don't know that a vaccine is that compelling. HIV is not the only infection you can contract from unprotected sex. If you have a stranger in front of you with whom without a vaccine you would have used a condom, would you have unprotected sex with them after having been inoculated? While it might make sense to get the vaccine anyway as a second layer of defense, in all likelihood it won't have any effect on your personal safety.