An experimental Zika vaccine completely protected mice after a single dose, according to new research from The University of Texas Medical Branch at Galveston and Instituto Evandro Chagas at the Ministry of Health in Brazil.

The vaccine is the first live-attenuated Zika vaccine, which means that it is developed using the actual virus that has been weakened in the laboratory to the point that it is harmless — or at least much less virulent than it was originally — but is still "live."

Zika has been recognized for about 70 years, and was known as a fairly mild illness, characterized by symptoms in healthy children and adults which include a mild fever, headache, and malaise, which lasts for two to seven days.

But the Zika virus can have devastating effects on the developing fetus. Pregnant women who are infected with the Zika virus but never display any disease symptoms may still give birth to a baby with microcephaly.  Microcephaly is characterized by a small head size and severe developmental problems.

Scientists have been racing to develop a Zika vaccine. These developing vaccines have been made from an inactivated version of the Zika virus, and some candidates have been shown effective in mice and nonhuman primates.

"We chose to pursue a vaccine made from live virus that has been sufficiently attenuated, or weakened, to be safe, and is able to illicit robust immune response to protect us from Zika virus infection. Such live-attenuated vaccine has the advantage of single-dose immunization, rapid and strong immune response and potentially long-lived protection," said UTMB's Pei-Yong Shi.

"A successful vaccine requires a fine balance between efficacy and safety — vaccines made from attenuated live viruses generally offer fast and durable immunity, but sometimes with the trade-off of reduced safety, whereas inactivated viruses often provide enhanced safety but may require several doses initially and periodic boosters," Shi said. "Therefore, a safe live-attenuated vaccine will be ideal in prevention of Zika virus infection, especially in developing countries."

To create the vaccine, the researchers engineered the Zika virus by deleting one segment of the viral genome. A similar approach has successfully been used to develop a dengue virus vaccine, which is currently in the final phase of human trials.

Shi explained that the Zika vaccine appears to have a good balance between safety and efficacy. A single immunization produced strong immune responses and prevented the virus from infecting mice at all.

"Safety is a major hurdle when developing a live-attenuated vaccine. Our Zika vaccine showed promising safety profile in mice when compared with clinically approved live-attenuated vaccines, such as the yellow fever vaccine," Shi said.

The study was published in Nature Medicine.