An experimental Zika vaccine has proven to be effective in protecting monkey fetuses, lending further encouragement to early-stage trials testing the safety and efficacy of the same vaccine in humans.
In 2016, the World Health Organization (WHO) declared the Zika virus a Public Health Emergency of International Concern due to its spread throughout Latin America and its association with congenital abnormalities in the infants of infected mothers. The virus is primarily transmitted through infected mosquitos but can be spread through sexual activities or sharing blood, such as the supply between mother and infant during pregnancy.
Though infected adults usually have no symptoms, virus infection in pregnant women is linked to a high risk of adverse fetal effects, including death, microcephaly (an unusually small head), and other abnormalities collectively known as congenital Zika syndrome. Currently, there is no treatment for the virus or its associated diseases, notes the WHO. Zika DNA vaccine VRC5283, along with several other related vaccines, is currently in clinical testing for human development, but global declines in Zika virus infection have made advancing vaccine development a lesser priority.
- "Zika is currently not much on the news due to the fading epidemic, because in many areas where outbreaks happened in 2015 and 2016, most people have become infected and have developed immunity; thus, there is high 'herd immunity,'" explained study author Koen Van Rompay to IFLScience. The VRC5283 vaccine was first described and tested in nonpregnant macaques and then moved to phase 1 human trials. It is currently in phase 2B clinical trials at 17 sites in eight countries with around 2,000 people enrolled.
Writing in Science Translational Medicine, researchers from the California National Primate Research Center at the University of California, Davis, and the National Institute of Allergy and Infectious Diseases say the monkey trial will help support the case for approving a vaccine in humans. Outside of a laboratory, a person may be immunized for months or years before encountering the pathogen. To mimic this real-world scenario, researchers vaccinated female macaques with VRC5283 before allowing them to procreate. In all, 13 vaccinated animals and 12 unvaccinated controls became pregnant before being exposed to the virus during their first and second trimesters.
After pregnancy, vaccinated females had lower virus levels present in their blood and improved fetal outcomes; two unvaccinated monkeys lost their fetuses in early pregnancy due to the infection, whereas no vaccinated animals lost theirs. Furthermore, 11 of the 12 offspring from unvaccinated monkeys had detectable Zika virus RNA present in their tissues, but the offspring of vaccinated monkeys did not, suggesting the vaccine may prevent mother-to-fetus transmission.
Macaques are biologically similar to humans, which means the safety and efficacy of new medications on the primates may result in a similar outcome in humans, according to the Max Planck Society.
"There are still many regions of the world where most people are sensitive, and where the mosquito vectors are present. In addition, even in regions with currently high herd immunity, this herd immunity will fade over time. Thus, it is only a matter of time before new outbreaks will happen. We should be prepared for this, especially so that we can protect pregnant women and their fetuses/newborns from the harmful effects of Zika virus infection," said Van Rompay.
Though the animal group sizes were relatively small, the researchers say their work provides a "proof of concept that a Zika virus can be effective during pregnancy" and gives hope for a future vaccine in humans.