Microcephaly case-control study confirms Zika virus causality

The relation between Zika virus and microcephaly is widely assumed to be causal because of strong evidence of an association. However, evidence so far comes from case reports, case series, modelling studies, and preliminary reports from cohort studies – none of which have included appropriate control groups. Preliminary findings from 32 cases now confirm causality, but the true size of the effect will only be available following the full analysis of all 200 cases and 400 controls, say scientists.