Preclinal tests have shown that a new mRNA-based Zika vaccine was able to offer protective benefits to mice and monkeys with just one dose.
In a study published in the journal Nature, Drew Weissman, M.D., Ph.D., and colleagues reported the vaccine candidate offering durable and rapid immunity against the Zika virus without adverse effects. Given the potential the vaccine has for addressing the global threat, the researchers said they are hoping to begin with clinical trials between 12 and 18 months from now.
There are several Zika vaccines in development but this is the first time that such a level of potency and longevity in protecting against the virus without using a live version of Zika was observed.
Where typical viral vaccines featured killed or weakened forms of the virus or isolated virus proteins, the candidate Zika vaccine utilizes tiny RNA strands holding genetic codes for viral proteins. The RNA molecules are altered forms of messenger RNAs carrying gene information and acting as blueprints for protein production in cells.
Any mRNA injected into the body would normally be cleared in a matter of minutes by the immune system but the mRNAs used in the candidate vaccine have been tweaked so that they are overlooked by the body's defenses. Once mRNAs are in cells, they are incorporated into the protein-making process, inducing production of the virus proteins they encode over a period of weeks.
Zika Vaccine Development
Viral proteins produced in an extended manner mimics the effects that vaccines using live viruses would get. But while live virus vaccines offer stronger immunity protection than vaccines that use non-replication viruses or isolated virus proteins, they have a serious drawback: harmful infections for people with weakened immunity.
Newer candidates for vaccines have opted for harmless viruses to deliver genes necessary for viral protein encoding. Formulations using a strategy based on adenoviruses are the only ones to show strong protective benefit for monkeys with just one dose, they have a tendency of being neutralized by the immune system as it attacks the adenoviruses before a vaccine candidate is able to deliver immunizing effects.
The current vaccine candidate using mRNAs don't have these drawbacks.
A Working Zika Vaccine
When the researchers tested the vaccine on mice, it resulted in a quick immune response that still protected animal subjects when they exposed to a separate strain of Zika after two weeks. A few days since the exposure, the virus was undetectable in the blood and protective benefit was extended even after five months since the vaccination.
One injection is potent enough even if it just measures just 30 millionth of a gram, which is a small fraction of a typical vaccine dose.
In the case of macaque monkeys, these subjects were given single vaccine doses of 50 micrograms each, which was enough to offer protection against Zika exposure five weeks later.
In both mice and monkey subjects, the researchers observed that the vaccine led to a surge in antibodies that blocked the Zika infection. Antibody levels reached their highest after several weeks and stayed high enough to offer protection, which the researchers believed had the potential to last for years.
"Our work so far suggests that this new vaccine strategy induces a level of virus neutralization about 25 times greater than one sees in standard vaccines," said Weissman, the study's senior author.
