Why the Zika outbreak might end with a whimper and other FAQs
The Zika virus has been in the headlines so often over the past few months that it's tempting to assume this new threat is here for good: That like dengue and West Nile virus, this mosquito-spread disease is now going to be a regular fixture — and a perennial risk for pregnant women.
But will it?
There are more questions than answers about what the future holds for the interplay between humans and the Zika virus. Let's take a look at some important ones:
Q: How long is Zika going to stick around? And how often will it return?
Disease experts sometimes refer to a virus's "periodicity" — how long it sticks around and the frequency with which it will return.
Very little is known about Zika's periodicity. Although it was discovered way back in 1947, before 2007 human infections were rarely seen, popping up only occasionally in parts of Africa and Asia. The huge bloom of Zika activity in the Americas right now is unprecedented.
STAT asked two of the world's leading experts on arboviruses — viruses transmitted by insects like mosquitoes and ticks — to look into their crystal balls to predict how Zika might behave in the future.
Duane Gubler, who teaches at the Duke-NUS Medical School in Singapore and who is a member of the World Health Organization's Zika emergency committee, assumes the virus will behave like dengue and other arboviruses, recurring regularly with upticks in cases during the rainy season in places that have one.
Dr. Scott Halstead sees the future differently — and he makes a bold prediction. A longtime US Army scientist who is now semiretired, Halstead believes Zika will vanish in a few years, likely for decades. That's the way another arbovirus, chikungunya works, he wrote in an article in Emerging Infectious Diseases last year.
"The rate at which people are getting infected right now certainly suggests that we're heading, within a year or two, to achieve sufficient herd immunity for this virus to die out," Halstead told STAT.
Both Halstead and Gubler agree that the outbreak currently raging in the Americas is due to the fact people of the region had no immunity to the virus; it's a so-called virgin soil outbreak. As a result, Zika is infecting people at a brisk clip. At some point, this type of explosive outbreak tends to burn out because many people become immune. But then what?
Q: Will future outbreaks be like this one?
If Gubler is right, future epidemics — at least in places that have already had them — may be smaller. It's an immunity thing: People who have been infected with the Zika virus may be safe from it for the rest of their lives. That's still a theory that needs to be studied, but it is the prevailing one. And it would mean outbreaks like the ones plaguing the Americas are immunizing a lot of people in one big rush.
If Halstead is right, huge outbreaks could happen, but at irregular and distant points in the future. Long lulls between outbreaks — decades even, as Halstead suggested — would allow for a buildup of a lot of younger people who aren't immune.
Q: If Zika is immunizing lots of people now, does that mean there might be fewer cases of microcephaly in a couple of years?
That, too, is unclear. But as the virus moves through affected countries, at least some young girls who haven't yet reached their reproductive years are getting infected. If the lifelong immunity theory is correct, these girls won't need to worry about Zika infections when they reach the age where they start having children.
In fact, some people have actually questioned whether mosquito control efforts might have an unintended consequence — preventing young girls from getting protected through infection. Norway's former chief epidemiologist, Preben Aavitsland, has raised the idea in a blog post. (See point 6 here.) He suggested advising parents to let young girls be bitten — even to the point of putting them in a room with infected mosquitoes, the Zika equivalent of a chickenpox party.
Others are dubious about that notion.
Dr. Beth Kirkpatrick, director of the University of Vermont's Vaccine Testing Center, said the natural infection approach is too sporadic and unpredictable. Some people who shouldn't get infected — pregnant women — will. And some of the people you hope will become infected — young girls — will not.
Q: Will Zika strike Africa and Asia with the ferocity with which it has battered the Americas?
Cape Verde, off the coast of Senegal, has had a recent Zika outbreak. But it's not yet known whether the outbreak was caused by the African strain of Zika that's been around for decades, or the Asian strain that's responsible for the epidemics in the Americas.
Identifying the culprit could tell us a lot about what might happen next. If it's the Asian strain — and if there's evidence the African strain had already circulated in Cape Verde — it might suggest Africa, at least, is vulnerable to the outbreak strain.
However, STAT spoke with a number of experts who said they expect the two strains to be cross-protective – meaning infection with one would protect against the other. "Zika is Zika is Zika,'' said Stephen Whitehead, who is working on a Zika vaccine at the National Institute of Allergy and Infectious Diseases.
Gubler agreed. Still, he doesn't believe there is widespread immunity to Zika in Africa and Asia. Sure, there probably have been more cases than the medical community recognized, misdiagnosing them as dengue, he said, but "they've not really had any major epidemics there."
Q: Does this uncertainty affect the prospects for a Zika vaccine?
In the early 2000s, when West Nile virus was new to North America, there was huge interest in a vaccine. But by the time US government scientists who designed one went looking for an industry partner to produce it, the interest, like concern over the virus, had abated.
There is no West Nile vaccine. And that could be a cautionary tale in the case of a Zika vaccine.
If Zika activity subsides, manufacturers will have a hard time conducting the clinical trials to prove the vaccines work. If no one is getting sick, how can you show a vaccine is protective? Without that kind of data, it is hard to persuade regulatory agencies like the Food and Drug Administration to license such products.
And vaccine manufacturers may question the commercial prospects of a vaccine aimed at preventing infection with a virus that isn't circulating.
"We need the vaccine right now. And the need for the vaccine probably will be a little bit less next year than it is this year. And then two years from now, the epidemic threat will disappear. Meanwhile, how are you going to get the vaccine licensed?" Halstead asked flatly.
Rajeev Venkayya sees things differently. A White House director of biodefense under President George W. Bush, Venkayya is president of the business unit for Takeda Pharmaceuticals, which is exploring possible partnerships for developing a Zika vaccine.
He thinks the alarm over the damage the virus inflicts on developing fetuses will continue to drive demand for a vaccine. "It will be a long time, I think, before this fear goes away."