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. Author manuscript; available in PMC: 2022 Dec 11.
Published in final edited form as: J Acquir Immune Defic Syndr. 2020 Jul 1;84(3):313–322. doi: 10.1097/QAI.0000000000002350

TABLE 1.

Sample Sizes and Corresponding Decision Rules Depending on the Number of Intrapartum Transmissions Observed at the Time of Interim Analyses

No. of Intrapartum Transmissions Observed
Interim Analyses Sample Size N = 0 N = 1 N = 2 N = 3 N = 4 N >4
1st 58 Continue Continue Stop for futility Stop for futility Stop for futility Stop for futility
2nd 118 Stop for efficacy Continue Continue Stop for futility Stop for futility Stop for futility
3rd 275 — Stop for efficacy Continue Continue Stop for futility Stop for futility
Final 410 — — Final success Final success Final success

Stopping criteria are met if there is either 80%-probability of futility (italic cells) or 95%-probability of efficacy (bold cells). Interim time-points were determined using Monte Carlo simulations, under a Beta-Binomial model with assumption based on the predictive probabilities to achieve either futility or efficacy at interim analyses.21 With a sample size of 410 mother–infant pairs receiving antiretroviral intensification, there was a 82% probability of obtaining the study results earlier, at one of the interim looks at N = 58, N = 118, or N = 275.