BETHESDA, MD - NOVEMBER 21: Nurse Floreliz Mendoza holds the syringe she has used to inject Andie Vaught, 25, with an experimental Zika vaccine as part of a clinical trial at the National Institute of Health Clinical Center in Bethesda, Maryland, November 21, 2016. This vaccine, one of about six or seven in different phases of clinical trial, is currently undergoing safety testing at three locations in the country. Instead of using dead or weakened virus to get the immune system to recognize the infection and fight it off, it uses a piece of DNA engineered to contain genes that code for proteins of the Zika virus. (Photo by Allison Shelley/For The Washington Post)