Zika captured the world's attention when microcephaly cases started popping up in Brazil back in 2015, and in a fast response, governments and industry quickly committed resources for vaccine research. But after the National Institute for Allergy and Infectious Diseases pushed a vaccine into phase 2 testing, the outbreak has subsided, making it harder to test for efficacy, Science reports. 

In response to those challenges, scientists are now considering pressing ahead with a human challenge study in which investigators would infect study participants with the virus to test vaccine efficacy. Johns Hopkins University Bloomberg School of Public Health professor Anna Durbin told Science her team plans to submit a new proposal for a challenge study in about a month.

An ethics panel previously blocked (PDF) such a challenge test, but in speaking with Science, the panel chair said she believes there’s now "compelling reason" for such a study. 

The study would be separate from the ongoing large NIAID trial, which is enrolling 2,500 participants to test a DNA vaccine in several countries including the U.S., Brazil, Peru, Costa Rica, Panama and Mexico.  

Even if the NIAID is unable to demonstrate efficacy through its current study, it’ll at least produce data on safety and immune responses. Director Anthony Fauci told Science the results, plus earlier data, could be enough to secure an FDA approval.

Zika popped up on the global stage in 2015, and the outbreak later slowed. In 2016, the World Health Organization said Zika was no longer an international emergency but that fighting Zika will continue to demand "intense action." 

NIAID and industry continued pushing ahead with their vaccines in preparation for the next outbreak, but those efforts have yielded at least one setback. Sanofi stepped away from its Zika vaccine research after its partner, the U.S. government, scaled back funding for the project. The decision followed a controversy over future pricing for the shot. Takeda, Moderna, Inovio are among other teams working on Zika vaccines. 

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