Zika: Experimental vaccine stops the virus from replicating in mice

Pregnant mice that were vaccinated before being exposed to Zika also showed signs of a healthier pregnancy than their unvaccinated counterparts

By Carissa Wong

An experimental vaccine against the mosquito borne Zika virus could prevent the potentially life-threatening infection from taking hold in pregnancy, a study in mice suggests.

Although usually mild, a Zika infection during pregnancy can stunt a fetus’ brain development and cause babies to be born with a head that is smaller than expected, known as microcephaly. The virus is also associated with other pregnancy complications, such as premature birth and miscarriage, and can be fatal in severe cases.

In 2016, the World Health Organization declared a Zika outbreak across the Americas a Public Health Emergency of International Concern, the highest alarm it can sound.

There is no approved vaccine against the virus. Vaithilingaraja Arumugaswami at the University of California in Los Angeles and his colleagues have created a vaccine candidate that contains genetically engineered copies of the Zika virus genome. The idea is the tweaked virus genome enters cells to produce immune-stimulating viral proteins but cannot replicate itself.

To put the vaccine to the test, the researchers injected 10 female mice with one dose, followed by a booster two weeks later. An additional nine female mice were left unvaccinated.

A week later, the team allowed the vaccinated and unvaccinated mice to mate, with all the mice becoming pregnant. Thirteen days into their pregnancies, they were all injected with a high dose of Zika virus.

Eight days after the mice were injected with the virus, the researchers found it had replicated in the unvaccinated mice, but not their vaccinated counterparts.

A third group of nine pregnant mice weren’t vaccinated against Zika or injected with the virus, but were instead given a saline solution. This group represented a mouse pregnancy that is unaffected by Zika or the experimental vaccine.

Twenty-ones days after conception, considered a full-term pregnancy for mice, the fetuses were weighed. Those from the vaccinated mice weighed a comparable amount to the fetuses of those given a saline solution. This weight was about twice that of the fetuses from the unvaccinated mice, with fetal weight indicating the overall health of the pregnancy.

In an additional finding, the spleens of the vaccinated mice contained considerably more protective immune cells called T-cells than the unvaccinated mice, suggesting that the vaccine induced an immune response.

“This is a promising vaccine candidate worthy of further study,” says Sarah George at Saint Louis University in Missouri.

The findings are important given that a Zika outbreak is becoming more likely, says Paulo Verardi at the University of Connecticut.

“The potential broadening of the distribution of mosquitoes due to climate change, and the ability of RNA viruses [including Zika] to evolve and adapt, as seen with SARS-CoV-2, indicates that another significant outbreak is possible, if not very likely,” says Verardi.

However, the vaccine needs to be tested in other animal models and in humans, says Verardi. Further tests will also need to check if the engineered virus in the vaccine could mutate to replicate itself, he says.

Journal reference: Microbiology Spectrum, DOI: 10.1128/spectrum.01137-22