Though scientists have been able to create vaccines for other viruses, developing a vaccine for HIV has been especially challenging. (In lieu of one, doctors mostly use preventative drugs like Truvada to help prevent contraction of the virus in uninfected people who are at high risk.)

But a vaccine could significantly taper infections worldwide. “Even a moderately effective vaccine would significantly decrease the burden of HIV disease over time,” said Fauci in a statement.

So why are HIV vaccines so elusive?

HIV behaves unlike most other viruses in some important ways. Usually, when a person is infected with a virus, their immune system creates antibodies that target the bug. That’s usually the starting point for researchers, who work to develop drugs that can imitate that process (but without causing the recipient to develop a full blown reaction to the virus). What’s tricky about HIV, however, is that when a person is infected with the virus, that same process of developing antibodies isn’t triggered.