Since Inovio became the first to test a Zika vaccine in humans, several government agencies and Big Pharmas have either entered the clinic or embarked on the quest. Despite the enormous effort, though, a vaccine for women of childbearing age may not be available before 2020, the World Health Organization estimates.

Margaret Chan, director-general of WHO, made the comment in a piece published one year after she declared Zika as a public health emergency of international concern. The agency lifted that warning level last November, stressing that a "robust longer-term" approach was warranted.

Inovio’s candidate, dubbed GLS-5700, is currently in phase 1, with a second round involving 160 participants underway in Puerto Rico. The National Institutes of Health enrolled 80 volunteers to test its candidate last August and planned to move into phase 2 in early 2017. The most recent group to enter the clinic was the Walter Reed Army Institute of Research, which started human trials of a Zika purified inactivated virus vaccine last November.

The potential $1 billion market and public health benefits have also attracted attention from Big Pharmas around the globe. Sanofi, with a $43 million grant from the Biomedical Advanced Research and Development Authority, is working with the U.S. Army’s Walter Reed and Brazil’s vaccine research institute Fiocruz on the ZPIV candidate. GlaxoSmithKline, from its vaccine R&D hub in Rockville, Maryland, has entered into a collaboration with the NIH to develop self-amplifying mRNA technology for usage in a Zika vaccine.

In fact, according to Chan, some 40 candidate vaccines are in the works. But even with all that attention, Chan figures "a vaccine judged safe enough for use in women of childbearing age may not be fully licensed before 2020."

Even though 2020 is still a few years out, that turnaround pace is still faster than most vaccine development cycles, and Chan described some recent innovative efforts as "promising."

The outbreak started in Brazil in 2015 and has since affected about 70 countries. Even though the mosquito-borne disease poses little threat to adults, it is notorious for causing a potentially deadly brain deformation in newborns, thus making women who are of childbearing age particularly important in stopping the epidemic.