As Zika virus outbreaks continue to rage in South and Central America, lapping at US borders, scientists are making significant strides toward an effective vaccine.

Two types of experimental Zika vaccines, a DNA vaccine and an inactivated virus vaccine, were each able to completely protect mice with one dose, researchers report Tuesday in Nature. The animal data—the first to be published for Zika vaccines—follows news last week that the Food and Drug Administration gave two companies the green light to test another Zika DNA vaccine in humans. The companies, not associated with the researchers behind today’s study, reported that they have done similar animal studies with their vaccine, but they didn’t publish the results.

With today’s animal data, researchers are hopeful about the fate of the vaccines. “The protection was striking,” Dan Barouch, a study coauthor and vaccine researcher at Harvard Medical School, said in a press briefing. “Of course we need to be cautious about extrapolating results from mice into humans,” he noted, but the strength of the findings “certainly raise optimism” that we’re on our way to a safe and effective vaccine.

In the study, Barouch and colleagues infected several types of mice with Zika strains from Brazil and Puerto Rico. While these viruses typically only cause mild symptoms in adults, they can cause rare cases of paralyzing neurological conditions and severe birth defects, such as microcephaly, when infecting pregnant women.

After a single dose of either a DNA-based vaccine or an inactivated Zika virus, mice were completely protected from Zika. Days after being infected, the mice had no detectable amounts of Zika virus in them, the researchers found. That protection lasted for at least two months. Unvaccinated control animals, meanwhile, showed high levels of virus in their blood.

Creating DNA-based vaccines and inactivated virus vaccines are two common strategies in vaccine development. The two companies starting the first human trials of a Zika vaccine, Inovio Pharmaceuticals, Inc. and GeneOne Life Science, also created a DNA-based vaccine. Generally, the DNA snippets included in such vaccines are viral blueprints for the unique proteins that cloak a particular virus particle. Smuggled into human cells, the code can be deciphered and used to train the immune system to spot the virus by its distinctive outerwear. DNA vaccines are being developed for a variety of diseases, including cancers and Ebola, but so far none has been approved for use in the US.

For inactivated vaccines, researchers use whole microbes, killed by heat, radiation, or chemicals (in this case by a chemical treatment) and offer them to the immune system for training. This strategy has worked for other illnesses, including polio, flu, cholera, and plague.

In the mice, both vaccines spurred the rodents’ immune systems to create ‘neutralizing antibodies’—proteins that could accurately identify Zika virus particles and raise immune responses strong enough to take out the virus. Next, the researchers will have to test the vaccines in more and bigger animals before moving on to humans.

Another possible hurdle is looking at humans who have been exposed to Dengue virus, a relative of Zika that also circulates in parts of South and Central America. On Monday, another study reported that antibodies from past dengue infections can also pick out Zika virus. However, sometimes the antibodies were neutralizing and sometimes they made the immune system go haywire, leading to more severe Zika infections.

Nature, 2016. DOI: 10.1038/nature18952 (About DOIs).