Just this week, it was announced that a new experimental vaccine for the Zika virus is set to go to human trials, suggesting our fight against the dangerous pathogen is turning a corner. However, as two new studies reveal, there is a lot about the Zika virus we are yet to fully understand, and its ability to replicate is more fearsome than we previously thought.

The first study, published in the journal Nature Immunology, highlights the fact that Zika seems to have a co-conspirator when it comes to infecting humans: dengue.

Both viruses are spread by the same mosquito, the Aedes genus – primarily the Aedes aegypti species. As Professor Gavin Screaton, the Dean of the Faculty of Medicine at Imperial College London, and the coordinating researcher of the study, told IFLScience: “It is possible that Zika will spread to the regions where the vector is commonly found,” as confirmed by many pre-existing studies.

In fact, the prevalence of dengue already exceeds 90 percent in some regions affected by Zika. This means that the likelihood of a dengue infection, followed by a Zika virus infection, can be fairly high in certain regions.

^{The same mosquito spreads both dengue and Zika. Tacio Philip Sansonovski/Shutterstock}

As this new study reveals, this is really bad news. By taking antibodies from people previously infected with dengue, and letting them interact with samples of the Zika virus, the team of researchers were able to see in real-time how the virus reacts to their presence, just as they would do in the bloodstreams of dengue-immune patients.

As it turns out, the majority of dengue antibodies – created by the immune system to recognize and neutralize the dengue should a repeat infection occur – are unable to pin down and stop the advance of the Zika virus. This is particularly remarkable, considering the fact that the two viruses are from the same viral family.

Some had thought that, because of this fact, the antibodies produced by the body would be almost identical after either infection, and that being infected with dengue may prepare the body somewhat for the arrival of Zika. Distressingly, this appears not to be valid.

Worse, it also appears that a prior infection of dengue actually strengthens the subsequent Zika virus infection by allowing the Zika to replicate far quicker – up to 12 times the speed it would otherwise.

This is because the immune system has been tricked. The dengue antibodies attempt to bind to the Zika virus, thinking it is still dengue. This triggers the release of white blood cells, who arrive to consume the Zika virus, assuming the antibodies have neutralized it. As they haven’t, the Zika virus then infects these incoming cells, and the infection spreads far quicker. This is a process known as antibody-dependent enhancement (ADE).

However, as an accompanying study in Nature shows, there may be hope. Two of these dengue antibodies actually manage to neutralize the Zika virus where all others fail. By examining tissue cultures infected first with dengue, then with Zika, a separate team of researchers observed that antibodies “anti-EDE1 mAb” and “anti-EDE2 mAb” successfully bind with the virus and allow white blood cells to destroy them.

^{Zika (illustrated here) and dengue, in a manner of speaking, are co-conspirators. Kateryna Kon/Shutterstock}

“The difference between the EDE antibodies and the others is that the EDE antibodies target a very specific site on the virus,” Felix Rey, the head of structural virology at the Pasteur Institute and the study’s coordinating author, told IFLScience. “We now know that we need to develop a vaccine to stimulate production of antibodies against this particular site."

“Our antibodies could be considered to be used to treat pregnant women at risk of contracting Zika,” he concluded.

Ultimately, the information provided by these two studies could be used to help develop a vaccine that can neutralize both dengue and Zika in one fell swoop.