BACKGROUND: Although Zika virus (ZIKV) infection is typically self-limiting, 
other associated complications such as congenital birth defects and the 
Guillain-Barré syndrome are well described. There are no approved vaccines 
against ZIKV infection.
METHODS: In this phase 1, open-label clinical trial, we evaluated the safety and 
immunogenicity of a synthetic, consensus DNA vaccine (GLS-5700) encoding the 
ZIKV premembrane and envelope proteins in two groups of 20 participants each. 
The participants received either 1 mg or 2 mg of vaccine intradermally, with 
each injection followed by electroporation (the use of a pulsed electric field 
to introduce the DNA sequence into cells) at baseline, 4 weeks, and 12 weeks.
RESULTS: The median age of the participants was 38 years, and 60% were women; 
78% were White and 22% Black; in addition, 30% were Hispanic. At the interim 
analysis at 14 weeks (i.e., after the third dose of vaccine), no serious adverse 
events were reported. Local reactions at the vaccination site (e.g., 
injection-site pain, redness, swelling, and itching) occurred in approximately 
50% of the participants. After the third dose of vaccine, binding antibodies (as 
measured on enzyme-linked immunosorbent assay) were detected in all the 
participants, with geometric mean titers of 1642 and 2871 in recipients of 1 mg 
and 2 mg of vaccine, respectively. Neutralizing antibodies developed in 62% of 
the samples on Vero-cell assay. On neuronal-cell assay, there was 90% inhibition 
of ZIKV infection in 70% of the serum samples and 50% inhibition in 95% of the 
samples. The intraperitoneal injection of postvaccination serum protected 103 of 
112 IFNAR knockout mice (bred with deletion of genes encoding interferon-α and 
interferon-β receptors) (92%) that were challenged with a lethal dose of 
ZIKV-PR209 strain; none of the mice receiving baseline serum survived the 
challenge. Survival was independent of the neutralization titer.
CONCLUSIONS: In this phase 1, open-label clinical trial, a DNA vaccine elicited 
anti-ZIKV immune responses. Further studies are needed to better evaluate the 
safety and efficacy of the vaccine. (Funded by GeneOne Life Science and others; 
ZIKA-001 ClinicalTrials.gov number, NCT02809443.).