An innovative vaccine made from genetic material protected lab animals from the Zika virus in experiments, scientists report, calling it a "promising" lead in fighting the threat to humans.
Rapid and durable protection
A single, low dose of the vaccine shielded mice exposed to Zika five months after the shot, they reported in the journal Nature.
Monkeys exposed five weeks after they were inoculated were also not affected by the virus.
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"We observed rapid and durable protective immunity without adverse events," said senior author Drew Weissman, a professor at the University of Pennsylvania School of Medicine.
"This candidate vaccine represents a promising strategy for the global fight against Zika virus."
Since Zika erupted on a large scale in mid-2015, more than 1.5 million people have been infected, mostly in Brazil and other countries in South America.
It is transmitted mainly by mosquitoes, but also through sex.
While the virus causes mild, flu-like symptoms in most people, pregnant women run the risk of giving birth to babies with severe brain damage.
Build-up of antibodies
The World Health Organisation declared a global health emergency in February last year, and declared it over in mid-November.
Several vaccines for Zika are under development, and a few are in the early phases of human trials.
Most vaccines are made from weakened or inert versions of the targeted virus, and seek to trigger a build-up of antibodies to project against full-strength, live virus.
The new vaccine, however, is based on a modified version of genetic material called messenger RNA (mRNA).
Normally the human body would flush out foreign RNA, but the vaccine developed by Weissman's team is designed to pass under the radar of the immune system and enter directly into human cells.
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Once there, the vaccine RNA is incorporated into the cell's protein-making machinery, where it mimics the action of a live virus vaccine – building up an immune response.
Trials within a year
The RNA-based vaccine has several potential advantages over rivals, the researchers said.
Delivered into the blood stream by fat-based nanoparticles, it can be injected just below the skin and is thus easy to administer.
Unlike other vaccines, it only requires one dose, conferring major practical advantages.
"If a vaccine is effective after just a single immunisation, the infrastructure needed for its administration can be much simpler," Weissman said in a statement.
Most importantly, perhaps, RNA-based vaccines are non-replicating, meaning they cannot integrate into the host's genetic blueprint, a safety concern that comes with live-virus vaccines.
Further tests will determine the vaccine's effects on pregnant animals and unborn foetuses.
Weissman said human clinical trials could start within a year.
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