However, Walensky notes that first-line anti-retrovirals - those medicine given to newly diagnosed patients that can stave off symptoms for years - are much cheaper than they were a decade ago. "Second-line regimens have come down quite a bit but not to the level of first-line and countries are having a hard time affording them and increasingly over time, people are going to fail first-line therapy and they're going second-line therapy and then, eventually, they're going to need third-line therapy, some of them."

According to Walensky, history has shown that drug prices can come down when international pressure is applied to drug makers. That would change the calculus for governments. But for now, she says, countries should focus on treating as many people as they can, as early as possible.

Her paper is published in the online journal PLoS Medicine.