The U.S. has approved the world’s only twice-a-year shot to prevent HIV, the first step in an anticipated global rollout that could protect millions – although it’s unclear how many in the U.S. and abroad will get access to the powerful new option.

Clinical trials have shown that six-monthly injections of lenacapavir are almost 100 percent protective against becoming infected with HIV. But big questions remain over the drug’s affordability.