A federally appointed ethics panel has rejected a proposal to deliberately infect people with Zika virus, a common practice known as a human challenge study.

It was denied because there was "substantial uncertainty about the risks to potential volunteers," as well as potential risk to sexual partners, fetuses, and other members of the community. There are currently four experimental Zika vaccines in the earliest stages of human testing, according to STAT, but there is concern that the outbreak will be receding by the time vaccines are ready for testing in Phase 3 trials.

The unknowns: The panel cited how much has been discovered about Zika in the last six months alone about transmission and side effects as a reason to hold back right now. Anthony Fauci, director of the NIH's NIAID, told STAT he would likely approve a study on Zika infection side effects first.

What's ruffling feathers: The search for a vaccine is hampered with this decision, and any research group that wants to prove a vaccine works will want to conduct large human trials.