> high-risk low-incidence diseases like this conducted

If you're asking 'how do we test the vaccine without _trying_ to give people HIV", one option is to give it to a lot of people, and record their outcomes over time.

But first you need to prove the vaccine isnt like, super toxic in humans. So you do animal analog tests, then a limited test on humans, aka "Phase 1 trials". A previous article summarizes:

> Moderna’s vaccines passed Phase I testing earlier this year, which involves testing for safety using only a handful of human volunteers. Phase II tests for a vaccine’s overall effectiveness, and with the move into Phase III, Moderna will be looking at its efficacy versus other prevention treatments currently on the market, such as pre-exposure prophylaxis, also known as PreP.

- https://www.them.us/story/moderna-begins-hiv-vaccine-trials-...

While HIV is a relatively low incidence in the general population globally, there are some specific regions and subpopulations in various countries at much higher risk. In short, they mostly test HIV vaccines on young, single gay men in the West and young, single heterosexuals in South Africa, Tanzania, etc.

I guess the same as in diseases like tuberculosis. I’ve just read an article where they studied immune responses in recent TB infections. The authors recruited subjects from South Africa. I think it’s not that hard to get them to join the trials since payout can be quite significant where such disease are widespread, especially compared to Europe or North America. And some areas have just outrageous HIV prevalence. Just sample a hundred people and you get a decent sample size.

I am just speculating, but in many diseases like rabies and chickenpox you can give the vaccine after exposure to help the immune system fight the infection. It's because the period between exposure and symptoms is quite long, often more than a month for rabies for example.

HIV can stay in the body for years before developing AIDS. So you can actually recruit people that have been tested positive recently and have not developed the disease.