The US Food and Drug Administration on Monday approved the first human trial of an experimental Zika vaccine, according to a joint announcement by the two companies behind the new therapy.

The companies, Inovio Pharmaceuticals, Inc., based in Pennsylvania, and GeneOne Life Science, Inc., based in South Korea, said that their DNA-based vaccine candidate, dubbed GLS-5700, will be given to 40 people in a phase I trial. The trial will start “in the next weeks,” the companies said, and could yield results later this year.

Inovio and GeneOne noted in their announcement that pre-clinical data from animal studies suggested that the vaccine could induce a strong immune response that might protect against mosquito-transmitted Zika. But, like all phase I trials, their upcoming human study will not test how effective the vaccine is at fighting off Zika virus but rather will test its safety and appropriate dosage levels. If the DNA-based vaccine is found to be safe, it will then move on to larger trials on efficacy that will take years to complete.

In general, DNA-based vaccines work by containing a snippet of genetic blueprints from the disease researchers are trying to protect against. In this case, the genetic material is from the Zika virus and codes for proteins that usually sit on the outside of the viral particle. Once the DNA is injected into a person, researchers can use electrical pulses to get human cells to take in the Zika DNA. Those cells can then read its code and start manufacturing proteins that resemble part of the virus. The Zika-like spare parts can prime the immune system to recognize and attack the virus if it ever invades.

For Zika, such a DNA-based vaccine may seem ideal over other vaccine strategies, such as vaccines that contain live but weakened viruses. Zika generally only causes mild illness in healthy adults, but it is known to cause birth defects in unborn fetuses. Thus pregnant women and women who might become pregnant are the key groups to vaccinate. And injecting those groups with a live virus may seem too risky.

Still, the Zika trials will have a lot to prove. Many companies have been working on DNA-based vaccines for years, trying to get them to work for a range of diseases. This includes Inovio and GeneOne, who had been working on DNA-based vaccines against several diseases, including HIV and some cancers, when Zika virus began rapidly spreading in South and Central America. But, so far, no DNA-based vaccine has ever been approved for use in the US.

Nevertheless, Inovio and GeneOne aren't the only ones holding out hope for getting the strategy to work for Zika. The National Institute of Allergy and Infectious Diseases has also cooked up a DNA-based vaccine and has promising animal results. And NIAID Director Anthony Fauci told STAT in an e-mail Monday that its vaccine is also close to human trials. “We had projected that we would start our Phase 1 trial in September; it now looks like we may be able to start in mid-August,” he wrote.