In the present study the association of the MHC class IB M1 haplotype with a worse outcome of the infection was apparent in the 2 control macaques carrying this haplotype, but not in the 4 class IB M1 positive vaccinated monkeys, suggesting that the Tat vaccine effect had successfully counteracted the detrimental impact of class IB M1 haplotype on the course of infection. Here, TAT is linked to infection.