A new Zika vaccine tested in animals has the potential to provide long-term protection against the virus with a single dose, scientists say.
"We observed rapid and durable protective immunity without adverse events, and so we think this candidate vaccine represents a promising strategy for the global fight against Zika virus," said senior author Drew Weissman, Professor at Perelman School of Medicine at University of Pennsylvania in the US.
Results of preclinical tests, reported in the journal Nature, showed promising immune responses in both mice and monkeys. "We hope to start clinical trials in 12 to 18 months," Weissman said.
Traditional viral vaccines contain a weakened or killed version of the virus or isolated viral proteins.
By contrast, the new Zika candidate vaccine uses tiny strands of RNA that hold the genetic codes for making viral proteins.
These RNA molecules are modified versions of the so-called messenger RNAs (mRNAs) that normally carry information from genes and serve as blueprints for the making of proteins within cells.
In this case, the mRNAs - produced and purified in a laboratory or biotech production facility - are delivered like a normal vaccine in an injection.
The new candidate vaccine contains mRNAs encoding two key proteins from a Zika virus strain isolated in a 2013 outbreak.
The researchers found that in mice, a single injection of 30 millionths of a gram of these mRNAs - a small fraction of the dose used for a typical vaccine - induced a rapid immune response, which protected mice from intravenous exposure to a separate Zika strain two weeks later.
That protection, resulting in zero detectable virus in the bloodstream a few days after exposure, was maintained even when the mice were exposed to Zika virus five months after vaccination, the researchers said.
Tests in macaque monkeys also showed that a single vaccine dose of only 50 micrograms provided strong protection against exposure to Zika virus five weeks later.
In both cases, virus neutralisation tests indicated that the vaccine induced high levels of antibodies that block Zika infection - levels that peaked after several weeks and thereafter remained high enough to be protective, potentially for years.
"Our work so far suggests that this new vaccine strategy induces a level of virus neutralization about 25 times greater, after a single dose, than one sees in standard vaccines," Weissman said.
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