Abstract

In response to our population-based study in which we followed up more than 7000 HIV-negative women over 5 years and failed to find evidence that concurrent sexual partnerships are an important driver of HIV incidence, James Shelton, Martina Morris, and Helen Epstein cite studies on HIV incidence in stable concordant HIV-negative partnerships as “direct evidence” for the concurrency hypothesis. However, these studies do not test the concurrency hypothesis (that concurrent sexual partnerships increase the rate of spread of HIV in a population) because they lack a meaningful counterfactual—ie, HIV incidence in people with the same total number of partners over the observation period but in serially monogamous partnerships. Furthermore, Morris and Epstein's claim that the concurrency hypothesis has already been tested and shown to hold runs counter to ongoing public debates on the topic,1, 2 other empirical data,3, 4, 5 and the results of a recent systematic review.6 The claim is also inconsistent with Morris and Epstein's call for a randomised controlled trial to test the hypothesis, which would not be ethically permissible if it had indeed already been shown that the hypothesis held true.