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[Correspondence] Trial size, HIV pre-exposure prophylaxis, and breastfeeding – Authors’ reply

We thank Sheila Bird for her interest in our trial.1 The effect sizes used for the sample size calculation for the trial1,2 stemmed from clinical information available before August, 2011, with an assumed range for mother-to-child transmission of 3% to 5% for the lamivudine group. The expected transmission rate of 1·0% to 2·5% in the lopinavir–ritonavir group was deemed to be of clinical significance. The observed transmission rate in the lamivudine group was less than half that expected,3 while the rate in the lopinavir–ritonavir group was within the expected range.