Two vaccines capable of protecting against Zika virus have been tested successfully in mice, US scientists announced this week.
Declared a public health emergency by the WHO in February 2016, Zika is believed to have already infected as many as a million people in Brazil.
Most of the concern focuses on the link with microcephaly in infants born to mothers infected with the virus during the early phase of pregnancy. However, the poor availability of tests for the infection, and the lack of a vaccine, have left officials in affected parts of the Americas with little to offer people apart from warnings to avoid being bitten by mosquitoes, which spread the infection.
Now that may be about to change thanks to Harvard scientist Dan Barouch and his colleagues, who published results this week in the journal Nature showing successful testing of two independent vaccines.
These vaccines work in different ways but both appear to block completely Zika infection in laboratory mice.
One of the vaccines is produced by growing the virus in cell culture before chemically deactivating it to render it safe, and then injecting it. This introduces the immune system to the components of the virus and triggers to production of protective antibodies in the blood.
The other vaccine is a more futuristic "DNA vaccine". To make this, the researchers prepared a short piece of DNA corresponding to the genetic instructions used by the Zika virus to produce its outer coat. Specifically these are structures called the pre-membrane and envelope. Injected into the bloodstream, these DNA messages are taken up by cells, which read the genetic recipe and produce the viral coat proteins, in turn driving the immune system to produce white blood cells and antibodies capable of neutralising Zika.
In both cases, mice challenged with Zika up to 8 weeks after vaccination were completely protected and the scientists were unable to recover any traces of Zika virus from the bloodstreams of the animals. Un-vaccinated mice, however, rapidly developed high levels of circulating virus.
"That the inactivated vaccine works is very encouraging," says Barouch, "because that method of making vaccines is tried, tested and trusted. That means that translation to the clinic can occur more rapidly."
Although the technology is not yet used clinically, the DNA vaccine approach taken by the team has a number of significant advantages over its rival.
The DNA message used can readily and rapidly be altered, for instance if the circulating virus were to mutate. It is also cheap to produce and easy to store, unlike traditionally-prepared vaccines which can require more careful handling.
However, a number of critical questions have not yet been answered in the study. The mice that the team tested were injected with Zika virus, rather than being infected via mosquito bites, which is the normal route of transmission.
"And that's why we need to be cautious in translating these results from mice to men," Barouch acknowledges.
Zika is also a close relative of the dengue virus, which circulates in the same geographies and is transmitted by the same mosquitoes.
The Harvard team haven't investigated whether their vaccine can still work in individuals who have previously been infected with dengue.
"It's possible that cross-reactivity between the immune responses against the two viruses might cause difficulties," Barouch cautions.
These unknowns aside, the announcement this week is a big step forward and welcome news for the millions in the flight path of the Zika spread.
The next stages will inevitably be to begin the process of translation of the work to humans, which will hopefully proceed rapidly...
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