Zika vaccine induces robust immune responses in three phase 1 trials

"This is the first report of the immunogenicity of the Zika purified inactivated virus vaccine -- or ZPIV -- in humans," said co-senior author Dan Barouch, MD, PhD, the Director of the Center for Virology and Vaccine Research at BIDMC and a Professor of Medicine at Harvard Medical School. "Previously, we demonstrated that ZPIV protected mice and monkeys against Zika virus."

The ZPIV vaccine candidate was developed by WRAIR as part of the US Department of Defense response to the 2015-2016 outbreak of Zika virus in the Americas, and it was advanced to a Phase 1 trial in November 2016. "The Zika epidemic underscored the need for the development of a safe and effective vaccine," said Dr. Barouch.

In the trials, a total of 67 healthy adult volunteers received two injections, four weeks apart: 55 received the ZPIV vaccine and 12 received a placebo. Investigators measured participants' immune responses by monitoring levels of Zika virus-neutralizing antibodies in the blood.

In nearly all individuals immunized with the investigative vaccine, ZPIV produced neutralizing antibody responses at two or four weeks after the last dose. By day 57, 92 percent of vaccine recipients mounted strong antibody responses against Zika virus, with levels of virus-neutralizing antibodies in the blood exceeding the protective thresholds observed in prior animal models.

When the researchers injected mice with antibodies from vaccinated people in the study, the animals were protected against subsequent exposure to Zika virus, unlike mice that were injected with antibodies from participants who received placebo. The investigators also noted that the vaccine was safe and caused only mild or moderate reactions.

"Zika virus has the potential to be like other viruses that are dangerous in pregnancy -- think measles, mumps or rubella -- that were knocked down significantly by effective vaccines," said co-lead author Kathryn Stephenson, MD, MPH, the Director of the Clinical Trials Unit at BIDMC's Center for Virology and Vaccine Research and an Assistant Professor of Medicine at Harvard Medical School. "These Phase 1 ZPIV studies are an important step towards bringing an effective Zika vaccine to fruition. Unfortunately, we continue to need these studies because the threat of a resurgent Zika outbreak is very real."

The trials are ongoing and differ in their design, with each hoping to answer separate questions regarding responses to ZPIV. The trial at BIDMC is evaluating three different ZPIV dosing schedules, while the trial at Saint Louis University is evaluating the safety and immunogenicity of three different vaccine doses. The trial conducted at the WRAIR Clinical Trial Center in Silver Spring, Md., is assessing the impact of pre-existing flavivirus immunity in volunteers who had been vaccinated against yellow fever or Japanese encephalitis -- both viral cousins to Zika -- prior to receiving ZPIV.