By examining both viral amino-acid sequences and the animals' immune responses, the scientists could determine the mechanisms of protection from SIV infection. The study demonstrated that antibodies to the virus spikes that SIV uses to infect cells are necessary and sufficient to prevent SIV infection. The study also identified clear measures of immune responses in monkeys that predict protection from SIV infection.

Amid the genetically heterogeneous mix of SIV to which the vaccinated monkeys were exposed, vaccine-induced immune responses tended to block infection by those viruses sensitive to neutralization by SIV antibodies, while neutralization-resistant forms of SIV tended to cause infection. A two-amino-acid change to the spikes on SIV converted neutralization-sensitive SIV to neutralization-resistant SIV, and vice versa. A similar change to the spikes on HIV had a related effect. Thus, SIV and HIV escape the immune system in similar ways, the scientists discovered. They concluded that the reasons why future human HIV vaccine trials fail or succeed will become clearer if scientists integrate information on the amino-acid sequence and neutralization sensitivity or resistance of the infecting virus together with information about volunteers' immune responses to the vaccine.