Scientists with the U.S. Department of Defense have launched a Phase I clinical trial to test an investigational Zika vaccine that relies on inactivated virus.

The candidate vaccine is known as the Zika Purified Inactivated Virus (ZPIV) vaccine and contains whole but inactivated Zika virus particles to stimulate an immune system response without replicating and causing illness.

The Walter Reed Army Institute of Research Clinical Trial Center in Silver Spring, Md., which successfully developed a vaccine for Japanese encephalitis in 2009, is hosting the first of five such early-phase clinical trials. The other Phase I trials of the ZPIV vaccine will be launching in the next few months.

“We urgently need a safe and effective vaccine to protect people from Zika virus infection as the virus continues to spread and cause serious public health consequences, particularly for pregnant women and their babies,” Anthony S. Fauci, MD , director of the National Institute of Allergy and Infectious Diseases (NIAID), said in a statement. “We are pleased to be part of the collaborative effort to advance this promising candidate vaccine into clinical trials.”

The NIAID helped support the preclinical development of the vaccine candidate and is part of a joint research agreement with the Department of Defense and other federal agencies to develop the vaccine for use in humans.

The trial at Walter Reed will enroll 75 individuals, ranging in age from 18-49 years, all of whom should have no history of a flavivirus infection. Twenty-five subjects will be given a pair of either intramuscular ZPIV injections, or a placebo (saline), with 28 days between injections.

The remaining 50 subjects will be divided into two groups of 25, with one group receiving two doses of a Japanese encephalitis virus vaccine and the other getting one dose of a yellow fever vaccine, before they both receive the two-dose ZPIV vaccine regimen.

A subgroup of 30 patients will then receive a third ZPIV dose 1 year later. Across all cohorts, the ZPIV dosage will be 5 micrograms.

In addition to the testing of the ZPIV vaccine, there are Phase I trials of a DNA-based Zika vaccine ongoing at the National Institutes of Health Clinical Center in Bethesda, Md., the Center for Vaccine Development at the University of Maryland in Baltimore, Md., and at Emory University in Atlanta, Ga. Those trials were launched in August 2016.