Zika virus (ZIKV) spreads to people primarily through bite by infected Aedes mosquitoes. ZIKV infection during pregnancy can cause stillbirths or affect the fetus by causing serious birth defects, such as microcephaly and other brain defects. Although uncommon, adults with ZIKV can also develop Guillain-Barre syndrome and other neurological disorders. According to the World Health Organization’s July 2019 report, a total of 87 countries and territories have had evidence of mosquito-borne transmission of ZIKV.

While Zika virus poses a substantial public health threat, no FDA-approved vaccine or treatment currently exists, and the only preventive measures for Zika virus involve mosquito population control or repellents. CDC and a partner co-developed an inactivated Zika virus vaccine candidate from a purified wild-type Zika virus isolate. CDC’s multiple plaque purification steps, genomic sequencing, and virus growth/kinetic analysis procedures enabled a more uniform, well-characterized, and clean virus stock without potential contaminants that were present in the original human virus isolate. Researchers used formalin and processes to ensure full virus inactivation, while retaining optimal viral particle structure and antigenicity. Studies showed that a single purified inactivated Zika vaccine (PIZV) candidate dose provided protection against ZIKV challenge in mice and rhesus macaques. In phase I clinical trials of healthy human volunteers not previously exposed to flaviviruses, intramuscular injections of the PIZV vaccine candidate were immunogenic, safe, and well tolerated up to 52 weeks of follow-up. Zika virus protection in humans was highest after two doses were administered. Biomedical Advanced Research and Development Authority (BARDA) funding supported this effort. Additional studies to optimize dosing schedules are ongoing.