Current status of Zika vaccine development: Zika vaccines advance into clinical evaluation

Zika virus (ZIKV), a mosquito-borne flavivirus, was first identified in the 1940s in Uganda in Africa and emerged in the Americas in Brazil in May 2015. In the 30 months since ZIKV emerged as a major public health problem, spectacular progress has been made with vaccine development cumulating with the publication of three reports of phase 1 clinical trials in the 4th quarter of 2017. Clinical trials involving candidate DNA and purified inactivated virus vaccines showed all were safe and well-tolerated in the small number of volunteers and all induced neutralizing antibodies, although these varied by vaccine candidate and dosing regimen. These results suggest that a Zika vaccine can be developed and that phase 2 clinical trials are warranted. However, it is difficult to compare the results from the different phase 1 studies or with neutralizing antibodies induced by licensed flavivirus vaccines (Japanese encephalitis, tick-borne encephalitis, and yellow fever) as neutralizing antibody assays vary and, unfortunately, there are no standards for Zika virus neutralizing antibodies. In addition to clinical studies, substantial progress continues to be made in nonclinical development, particularly in terms of the ability of candidate vaccines to protect reproductive tissues, and the potential use of monoclonal antibodies for passive prophylaxis.