The Aedes mosquito that carries the zika virus – as well as the dengue, yellow fever, West Nile fever and chikungunya flaviviruses – is so widespread that nearly a third of the world’s population is potentially at risk.
And yet there is no tablet or injection to treat or prevent a zika virus infection. The most that people can do is to try and avoid being bitten, as the authorities use insecticides to kill as many of the mosquitoes as possible.
While researchers and pharmaceutical companies race to find a way to out-fox the zika virus, any vaccine development is in its early days.
Nevertheless there is some important groundwork that can be built upon from other flavivirus vaccines such as for yellow fever and dengue.
Cameron Simmons from the Doherty Institute at the University of Melbourne and Oxford University heads a research group actively looking to control and ultimately to eliminate dengue.
He acknowledges the difficulties facing the task for zika.
“Making successful vaccines requires large investments of money and expertise from the private sector,” he says.
Nevertheless, he is upbeat that there will be a vaccine and relatively soon.
“The Ebola vaccine experience has taught us that vaccine development can be fast tracked if we’re determined, innovative and focused.
“While zika is not the same as Ebola, there are good reasons to be optimistic that a vaccine could receive regulatory approval in three to five years.”
How a vaccine would work?
Any zika vaccine would be one of three main types.
First, a live-attenuated vaccine might be the way.
This is where the virus is genetically altered so that it doesn’t cause disease, but being “live” it can still produce a robust immune response.
The risk is that the virus may be altered so much that it results in a weak immune response, or too little so that the virus isn’t attenuated and so signs and symptoms of zika develop after the vaccine is administered.
This strategy can take many years to develop and optimise.
A second approach would be to piggy-back zika antigens onto the technology used for the very widely accepted yellow fever vaccine.
This would mean splicing in the genes encoding the zika envelope proteins into the live attenuated yellow fever virus vaccine.
The advantage of this approach is that the safety is well established, easing the way for faster regulatory approval.
The vaccine company Sanofi Pasteur has used the ever-changing YF 17D strain of yellow fever virus to make vaccines against dengue and Japanese encephalitis.
Simmons, who has studied dengue virus populations after the introduction of the vaccine, believes that this approach has a good chance of success.
A third way is to construct what is called “a sub-unit vaccine”, which introduces an antigen to the immune system without introducing the whole virus.
This technology tends to be safer as there is no live virus in the mix.
The drawback is that finding the correct bits of the zika virus to stimulate an immune response is a long, painstaking and expensive task.
The response to a single component of the virus on its own is often too weak without using immune-stimulating agents, called adjuvants.
These are used in some influenza vaccines and in one vaccine against human papilloma virus.
How would we deploy a vaccine?
Any of these methods will take time, money and an enormous scientific effort.
And, after all that, when there is a vaccine available on the shelf that offers protection and is safe there remains the question – how should any vaccine be used?
Who should be given priority? Should it be given to children? To everyone?
“Clearly, a high priority is to stop pregnant women from getting zika virus infection,” says Simmons.
Yet these are a vulnerable group in who it is difficult to perform vaccine trials, as live vaccines are not recommended in pregnant women.
“So perhaps the best way is to immunise women of child-bearing age and reduce the number of infections in this population.”
While scientists work on the vaccine, there is much information still to be gathered that will determine the best response to these questions.
First we need to understand the epidemiology of the disease.
If zika burns brightly then fades away for decades then vaccination of a whole population may not be the right way to go.
If the virus turns out to be more like dengue and have a large background rate of infection, with periodic epidemics, then a vaccine will be very useful.
But at this stage we do not know.
Adam Jenney is an infectious disease physician and clinical microbiologist.
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