Intrarectal (IR) route with 10 TCID50 of SHIVAD8-EO, in the absence of antibody treatment. As shown in Fig. 1a, plasma viremia became detectable following 2 to 6 challenges, with a median of 3.0 weekly virus exposures needed to infect all nine animals. Based on these results, the inoculum size administered to each monkey per challenge was estimated to be 0.27 AID50. The regimen used to assess the