The full article would identify the test population. The study population referenced in the article is quite small (N = 230).
While "the poor in India or South America" make for a very colorful picture of the sort of test population unscrupulous researchers would go for, but if you think about it at all critically, you'll see that they're pretty unlikely for logistical reasons. The study involved participants receiving multiple injections over either 2 or 6 months, and blood tests at some point after each injection. With that schedule of office visits, life is much easier if your research subjects have phones and mailing addresses you can use to contact them, and if they can easily reach the clinic using public transit. Many of the people living in the slums of Delhi (just for example) are homeless, and the problem with homeless research subjects is that it's really hard to track them down if, for whatever reason, they don't come in. Since much of those Delhi slums are lacking sanitation and power, the clinic has to be elsewhere (need refrigeration for vaccines and blood samples, and clinic staff need to be able to wash their hands), so the study subjects will have to go out of their way for appointments - this is a recipe for patient attrition. If you offered money, you could probably recruit a study population in seconds (the IRB at your home institution would very likely disapprove this plan for ethical reasons, and could prevent you from continuing the research). But you'd have a lot of trouble getting them back for the second visit.
You'd want to do a study like this in an area with low prevalence of HepB in the population, so that you could be relatively confident that the antibodies you saw weren't the result of prior exposure to the disease. India and South America are bad for that. The poorer the subject population, the more bad they are.
If I were to place bets, I'd say that the study population were people from Rochester, MN or Emeryville, CA, recruited by ads in the paper.