Trial version in US giving successful immunisation to mice could help fight disease, but complications warned for those who have contracted dengue fever

An experimental vaccine that completely protects animals from the Zika virus has raised hopes for a jab that can bring the fast-spreading disease under control.

Trials in the US found that a single immunisation shot made from a purified and inactivated form of the Zika virus gave mice total protection against the illness that has swept through Brazil and other parts of South America.



The race is now on to convert this early success into an effective human vaccine, but that may not be straightforward. The similarity of the Zika virus to others in circulation means the vaccine may prove less effective in some people, and could potentially make other infections, such as dengue fever, more serious. Those concerns will almost certainly have to be addressed before human trials can begin.



Dan Barouch, who led the study at Beth Israel Deaconess Medical Center in Boston, called the trial results “a step forwards in the development of a Zika virus vaccine”, but said more research lay ahead. “Of course we need to be cautious about extrapolating results from mice into humans,” he said.



The vaccine is one of a number that have been under development as a top priority since the World Health Organisation in February declared the Zika epidemic a global public health emergency. Mosquitoes in more than 60 countries now carry the virus linked to severe birth defects and a paralysing neurological disorder called Guillain-Barré syndrome.



Writing in the journal, Nature, Barouch and colleagues describe how they tested two experimental Zika vaccines in mice. The first, known as a DNA vaccine, was made from genetic material taken from Zika virus circulating in the Brazil outbreak. The second was a more conventional vaccine, made from whole inactivated Zika virus in Puerto Rico. Both vaccines protected mice for at least two months.



“We were very surprised and quite impressed that a single shot of either one of these vaccines provided complete protection,” said Barouch. Tests are now underway to understand how long the vaccines are effective for, and whether boosters can extend the protection they offer.

Human testing of experimental Zika vaccine to begin Read more

Scientists have found it difficult to make DNA vaccines work in humans and none have ever been approved for use by the US regulatory authorities. For this reason, the second vaccine may prove easier to bring to market because it draws on conventional vaccine technology.



However, significant hurdles remain. Last week, researchers at Imperial College in London showed that previous exposure to dengue virus could potentially worsen Zika infections. If the reverse is also true, as some scientists suspect, a vaccine that causes the immune system to produce antibodies against Zika virus could inadvertently make dengue infections more life-threatening.

“We know that dengue circulates in the same parts of the world as Zika, and what you don’t want to do is go around and give a vaccine that makes another viral infection much worse and potentially fatal,” said Jonathan Ball, professor of molecular virology at the University of Nottingham.



The problem arises because the Zika and dengue viruses are so genetically similar that the immune system cannot always differentiate between them. When a person who has previously had dengue catches the Zika virus, the immune system may attempt to fight the new infection with “old” antibodies raised against dengue virus. Instead of wiping out the invading Zika viruses, the antibodies merely latch on and draw them into cells, helping them to replicate and spread.

For the same reason, a person who has had dengue in the past may have antibodies in their system that attack the Zika vaccine as soon as it is administered. If that happened, their body might swiftly wipe out the vaccine before it has a chance to make fresh antibodies against Zika virus.

Despite the hurdles, Ball said the results are still encouraging. Other vaccines that target flavivirus, the group of viruses that include Zika and dengue, are already effective, notably for yellow fever, and for Japanese encephalitis virus and the tick-borne encephalitis virus, which can cause the brain to swell.

“These studies are a good step forward and give reason to be optimistic that vaccines might work in people,” said Peter Openshaw, professor of experimental medicine at Imperial College London. “However, it is essential to move to human studies as soon as possible. By the time human vaccines are ready, many of the vulnerable population will have already been naturally infected. The purpose of vaccination will presumably be to protect travellers and those wishing to become pregnant. It will be vital to see how vaccines will work in such situations and how the practical and economic barriers to vaccine deployment can be overcome.”