A commentary cautions that though these trials are promising, further research is needed to address important questions. It notes one concern is that Zika virus has induced Guillain-Barré syndrome in around 1 in 4000 otherwise healthy infected adults. It suggests this effect could reflect autoimmune cross-reactivity of viral antigens and host molecules, and if that is the case, these viral antigens might need to be excluded from the vaccine formulation, and full understanding of the mechanism of Guillain-Barré syndrome associated with Zika virus could be crucial to ensure vaccine safety. It adds that another point of concern is whether immunity to Zika virus induced by vaccination will alter the behaviour of other flaviviruses, such as replication of dengue viruses or live dengue vaccines, and affect clinical and epidemiological outcomes. Lastly, field efficacy testing of Zika vaccines could be problematic.