The real problem with the monkey models is that experiments to evaluate vaccine efficacy requires challenging the vaccinated animals with high doses of virus to get 100% infection, that is, 10(3)-10(5) TCID50 equivalent to 5×10(7) SIV RNA copies/ml. This is in marked contrast to natural exposure to HIV in humans wherein the doses of 10(3) HIV RNA copies/ml of seminal plasma have been reported. However, these results are quite different in mucosal challenge, wherein reported low doses (10-30 TCID50) with SIV results in the same viral and immunological kinetics of infection as high-dose challenges. Thus, this new type of mucosal challenge might change the results of preclinical vaccine efficacy studies in the future.