On Tuesday, J&J, one of the companies working on the HIV vaccine announced that it had fully enrolled one phase 1/2a trial. It has recruited 400 volunteers from South Africa, Rwanda, Uganda, Thailand, and the US for this trial.

The company on also said it had started enrolling a second phase 1/2a trial. The vaccine will be given as a series of injections over the course of a year. The goal of these earlier trials will be to figure out if the vaccine is safe, and if so, what’s the best way to administer the vaccine.

Right now, Stoffels said, it’s too early to predict what an HIV vaccine would look like: It could be just one set of shots you get at one point in your life, or it is possible you might need follow-up booster shorts later down the line.

“That would take more research, to figure out if years from now would we have to boost,” he said. “It’s not impossible, but at the moment, it’s too early to predict.”

For now, the important thing is to keep plugging away at these earlier-stage trials that will judge the vaccines’ safety and tolerability as well as its ability to provoke an immune response. If that works out, companies can begin to think about what a late-stage trial that gears up for an approval will look like.