A new approach, published in Nature, outlines a vaccine that is successful at protecting mice and rhesus macaques from Zika virus. The vaccine is based on delivering viral mRNA to cells to induce an immune response and build protection.
Since its highly publicised outbreak in South America around the time of the 2016 Rio Olympics, Zika has continued to be a threat in 70 countries spanning five continents: Africa, North America, South America, Asia, and Australasia.
Zika virus is commonly spread by Aedes mosquitos, but can also be transmitted sexually. It is known to cause brain damage and microcephaly (abnormally small heads) in babies, and some researchers believe that there may be more subtle, longer term effects that we don’t know about yet.
The World Health Organisation (WHO) has stated there is significant evidence to suggest that Zika is also a cause of an autoimmune disease which attacks the nervous system, called Guillain-Barré syndrome.
However, Zika is no longer labelled a public health emergency, as it has become a long-term concern rather than an acute one. Using mRNA in the vaccine removes the risk of injecting the live virus into test patients.
There were several hurdles to the use of mRNA in a vaccine. First, mRNA injected into tissue is quickly broken down by the body’s enzymes. To overcome this problem, the mRNA is encased in lipid nanoparticles that, when injected, fuse with cell membranes to release the mRNA into the cell.
Once inside the cell mRNA is vulnerable to attack from the cell’s immune response before it has been able to transcribe any proteins (a process necessary for immunity). To overcome this issue, the team substituted one of the nucleotide bases in the mRNA with a modified base that allowed it to avoid immune detection until after it had been transcribed.
Where the other vaccines require two, this new vaccine only requires one injection to confer protection. Four out of the five monkeys in the study were protected for at least five weeks post-injection.
As the vaccine has had such promising results in its translation from mouse to macaque, it is expected to have a similarly high success rate in humans. Though promising in theory, as the team aim to be running clinical trials on humans within 18 months, it may be a while yet before any vaccine is rolled out.
Thus far, only one disease, smallpox, has been eradicated through the use of vaccines. However, this measure is thought to save approximately five million lives each year. Polio could be the next disease to disappear as a result of vaccination. A new preventative therapy could do the same for Zika.
Moderna Therapeutics, a biotech company working with the US Department of Health, has begun trials on a vaccine with a similar design, although they have not published any of their data.
Image: Manuel Almagro Rivas