To date, just one HIV vaccine trial has managed to demonstrate only moderate effectiveness: the six-year Thai study that, in 2009, showed a 31 percent reduction in infection rates, short of the necessary 50 percent benchmark required to slow the epidemic. A new investigation of the trial, however, shows promise for future avenues in research. It appears that the vaccine prompted four separate antibodies to mark the virus on a key site on the surface of HIV-infected cells, alerting “killer T-cells” to attack and kill them. Researchers say they cannot be sure this reaction was responsible for preventing transmissions, but when coupled with the recent discoveries about how the immune system’s “broadly neutralizing antibodies” can target the virus, these new insights may lead to a better vaccine down the road.