Zika vaccine shows promising results in animal studies

A Zika virus vaccine candidate, developed by the US-based Walter Reed Army Institute of Research (WRAIR), has been shown to be effective at preventing the Zika virus from passing from mother to foetus in preclinical animal studies. WRAIR collaborated with the Trudeau Institute and the Texas Biomedical Research Institute’s Southwest National Primate Research Center (SNPRC) on the study, which has been published in the journal npj Vaccines.

The 2015–2016 Zika outbreak in Brazil and other countries in the Americas caused a surge in miscarriages and a constellation of birth defects, called congenital Zika syndrome, including abnormally small heads and neurodevelopmental disorders. This prompted the World Health Organization to declare the Zika outbreak a public health emergency of international concern — an emergency that the study authors do not want to see repeated.

“It’s important to test vaccines before the next large outbreak, because there will be another,” said Dr Jean Patterson, a virologist at Texas Biomed and senior author on the new study. This led the team at WRAIR to develop their purified, inactivated Zika vaccine (ZPIV) candidate using the same technology they used to develop a Japanese encephalitis vaccine.

In Phase 1 human trials, the vaccine has been shown to be safe and to elicit a protective immune response. However, running rigorous tests to prove the vaccine protects pregnant women and their foetuses from both infection and severe malformations is restricted due to ethical and safety reasons. That’s where animal models come in.

Trudeau Institute and Texas Biomed researchers evaluated the vaccine in pregnant mice and marmosets, respectively. The mouse studies demonstrated the vaccine prevented about 80% of foetal malformations, and antibodies capable of neutralising the virus were detected in foetal blood samples eight days after infection.

“We were able to detect maternal antibodies in the foetus during pregnancy and the results suggest the antibodies play a critical role in protecting foetuses from Zika virus,” said Dr In-Jeong Kim, a viral immunologist at the Trudeau Institute and the first author on the paper.

Marmosets, which are small primates, are more sensitive to Zika infection than other nonhuman primates; previous studies showed foetuses were aborted within two weeks of maternal infection. In the current study, four marmosets were immunised with the ZPIV vaccine. After they became pregnant, they were exposed to Zika virus. Only one of 12 offspring tested positive for Zika virus, demonstrating more than 90% effectiveness.

“Because the animals became pregnant at different times, our study was able to show the vaccine confers protection for at least 18 months after vaccination, which is important for showing long-lasting immunity,” Dr Patterson said.

“Our proof-of-concept studies conducted at Trudeau and Texas Biomed show very promising results that the vaccine given before pregnancy will provide high levels of protection for mothers and babies,” Dr Kim concluded. The researchers are now testing what happens when the vaccine is administered during pregnancy.

Image caption: A vial of the Zika virus vaccine developed by a team at the Walter Reed Army Institute of Research. Image credit: WRAIR.

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