A Zika virus vaccine was injected into 75 healthy adults on Monday at the start of human trials of the vaccine developed by the Army, Defense Department officials announced Tuesday.

The vaccine was developed by scientists at the Walter Reed Army Institute of Research as part of the DoD response to the ongoing Zika outbreak in North America, South America and Southeast Asia.

The Zika virus is transmitted to humans primarily through mosquito bites, specifically from infected mosquitoes of the Aedes aegypti and Aedes albopictus species.

Military service members may be at risk for Zika infection during deployments, travel, and at military installations located in the southern U.S., where the climate and mosquito populations can create conditions for spread of the virus.

As of Nov. 2, 149 cases of Zika infection had been confirmed within the U.S. military health system, according to the Centers for Disease Control and Prevention. Of those, five women are pregnant: four service members and one spouse.  

Zika infection during pregnancy can lead to severe brain defects for the unborn baby, including microcephaly, eye and hearing problems, impaired growth and Guillain-Barre syndrome affecting the nervous system.

The first phase of human trials of the vaccine will assess its safety and ability to trigger an immune response. The tests, conducted at the institute's facility at Silver Spring, Maryland, will use a purified and inactivated vaccine called ZPIV, according to the DoD release.

This is the first of four Phase 1 trials set to begin this year, according to the release. The other three trials will look at the optimal dose to use, and evaluate the vaccine's safety and immune response both in a compressed schedule of vaccinations and in participants who have already been exposed to Zika or dengue viruses.

The Army has so far found ZPIV to be effective in mice and monkeys, Army Col. (Dr.) Nelson Michael said in a statement. Michael is director of the institute's Military HIV Research Program and Zika program co-lead.

The Army's response to the Zika virus was rapid, said Michael and Zika program co-lead Dr. Kayvon Modjarrad, moving in 10 months from recognizing the Zika virus as a threat to producing ZPIV for human testing and beginning clinical trials.

"A safe and effective Zika Vaccine that prevents infection in those at risk is a global public-health priority," said Army Maj. (Dr.) Leyi Lin, Zika study principal investigator, said in the release.

WRAIR is doing the research in collaboration with the National Institute of Allergy and Infectious Diseases and the Department of Health and Human Services' Biomedical Advanced Research and Development Authority.  

Walter Reed's ZPIV will be part of an NIH trial that began this summer, and the vaccine will be administered to volunteers as a booster after they are given an NIH vaccine that prompts an immune response.  

Other viruses

The Zika virus is one in a group called flaviviruses, which cause illness and death around the world.  They include yellow fever, dengue fever, Japanese encephalitis and West Nile viruses, according to the CDC.

Part of the assessment of the Zika vaccine involves its effect on viruses in this group, Lin said.

People who have a prior flavivirus infection or vaccination may become more severely ill when infected with another of the viruses, Lin said.

A priority is protecting service members who are vaccinated against flaviviruses and deployed or stationed in an area prone to Zika, the release said.

"Our study assesses co-vaccination to learn how to reduce risk when protecting against circulating flaviviruses," Lin said.