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[Correspondence] Association between Guillain-Barré syndrome and Zika virus infection

We commend Van-Mai Cao-Lormeau and colleagues (April 9, p 1531)1 for their recent work. However, several elements in their study design raise concern. In epidemiological terms, sampling of all incident Guillain-Barré cases in French Polynesia in a specific time period, as done by Cao-Lormeau and colleagues, constitutes sampling from a primary study base.2 In this context, controls should be a random sample (or stratified random sample) of the entire non-case portion of the base population.2 The authors, however, restricted control group 1 to individuals with non-febrile illness.