Work is progressing on developing a vaccine that can thwart the Zika virus, but it will take years to get one to market, experts warn.
Millions of doses needed
Two rival vaccines for Zika have the best chance of becoming available for field testing by early next year, said Dr Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases (NIAID).
But testing the vaccines could become complicated if Zika does not reappear in the South American countries where it achieved epidemic proportions this year, Fauci wrote in a commentary published in the New England Journal of Medicine.
Testing could also be hampered if vaccine manufacturers aren't able to quickly make the millions of doses needed for field trials, added Dr Stephen Thomas, an infectious diseases doctor at SUNY Upstate Medical University in Syracuse, New York. He also wrote a commentary in the same journal.
Read: 5 things to know about the Zika virus
"The challenge is not just showing that it is safe and has the clinical benefit you want, but making enough doses and deploying it," said Thomas. He recently moved to SUNY after serving as the Zika programme lead for the Walter Reed Army Institute of Research in Silver Spring, Maryland. "There are a lot of very complex issues at play here," he said.
The earliest scientists will know whether a Zika vaccine works is early- to mid-2018, Fauci said. All told, it could take three to five years to have an effective vaccine to use in areas of active Zika transmission, Thomas added.
No microcephaly in Florida
Zika is the first mosquito-borne virus known to cause terrible birth defects, most of them brain-related. The most common defect is microcephaly, in which a child is born with an abnormally small brain and skull. Thousands of babies have been born with Zika-linked microcephaly, most of them in Brazil, since an outbreak began in South America in April 2015.
Zika infections began occurring in south Florida this summer, with 43 cases as of Sept. 21, according to the US Centres for Disease Control and Prevention. There have been no reports yet of microcephaly in the state.
NIAID has developed an experimental DNA-based vaccine that recently entered human safety trials. At the same time, Walter Reed is collaborating with pharmaceutical manufacturer Sanofi Pasteur to develop a vaccine based on inactivated Zika virus, Fauci said.
Both vaccines should be ready for field testing by January, when summer in South America rolls around, Fauci added.
Read: Brain infection also associated with Zika virus
Public health experts believe that Zika will flare up again with the coming of the summer mosquito season, which will provide an opportunity to test the vaccines and gather valuable data on effectiveness, he said.
"We don't think the outbreak is going to disappear in a year, the way Ebola did," Fauci said. "Zika likely will hang around in South America and the Caribbean and Puerto Rico for more than one season. We hope it doesn't, but if it does we will be ready to start a phase 2 trial."
Regardless, public health officials also are considering alternative plans for testing the vaccines, should Zika outbreaks fail to appear, Fauci said.
Candidate vaccines will be tested using the "animal rule", he explained.
"If you vaccinate a non-human primate and you create an immune response that protects the animal, then you go to humans and do a lot of studies of safety," Fauci said.
"You can include thousands of people, even though there's no infection in society, and you measure what their immune response is. If the immune response is comparable to what the animal got when the animal was protected, then you can make an assumption that immune response would also protect a human," he said.
Read: Don't be fooled by 'mild' Zika virus
Human challenge trials are another way to test a Zika vaccine. Half of a group of healthy volunteers would receive the Zika vaccine, and half would not. All then would be exposed to Zika virus, Fauci said.
"If you protect those who are vaccinated but don't protect those who are not vaccinated, you can assume the vaccine works," Fauci said. "That is much quicker than doing a field trial, which may take a year or more. With this, you could find out the answer in a month or so."
However, there are safety considerations in human challenge trials, Fauci said.
Zika is a very mild virus, causing no symptoms in four out of five people. It mainly poses a danger to the unborn, as an infection in a pregnant woman can lead to devastating birth defects.
Read: 10 facts you should know about Zika
But the virus has also been linked to rare cases of Guillain-Barre syndrome, in which people experience muscle weakness and paralysis, Fauci noted.
"You have to make sure you give it a good ethical review before you challenge a human," Fauci said. "You have to make sure you have all of your ducks in order, in terms of informed consent."
Those plans assume that a candidate vaccine is available in quantities for testing, which is not a firm assumption, Thomas said in his commentary.
Production would have to scale up from 1,500 doses made in a research lab for initial testing to millions of doses churned out by a state-of the art manufacturing facility, he said.
"Now everything is different," Thomas said. "It's like cooking for the family versus cooking for the cruise ship.
"It is going to require a bunch of different groups to come together and make this thing work, but there are a lot of reasons to be optimistic that it can be done," he said.
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