ast summer, I received a telephone call from a relative who was terrified about Zika. A young woman in her early thirties, she was pregnant with her first child and was slated to travel to South Florida. At the time, the news was filled with stories of infected infants who had been born with shrunken heads and damaged brains—a condition known as microcephaly—and whose futures would be marked by severe debility. The Zika virus had spread to Florida from South and Central America; the pathogen was transmitted mainly by mosquitoes but could also be passed through sexual intercourse. There had already been several hundred cases of local transmission in the continental United States. The World Health Organization had declared Zika a global emergency.
While my relative’s risk was low, her concerns were real, and she cancelled the trip. She was not alone in avoiding epicenters of the burgeoning epidemic. Even some Olympic athletes, who had spent years training, chose not to attend the Summer Games in Brazil, where there would be more than a million documented cases of infection and some thirty-five hundred infants born with microcephaly. Now, however, the spread of Zika in the continental United States has virtually ended. As September begins, there have been only two cases of local transmission reported in 2017—one in Hidalgo County, Texas, near the Mexican border. Dramatic declines in the numbers of new Zika cases have also been noted in South and Central America. How did the epidemic explode in the Americas and then withdraw so quickly? And is Zika gone for good?
Zika belongs to the genus Flavivirus, which includes yellow fever and dengue. It was originally identified seventy years ago, in monkeys in Uganda’s Zika Forest, and was contained for decades to a narrow equatorial belt in Africa. With today’s global travel—a common vector for transporting epidemics—Zika arrived in Micronesia, in 2007, infecting nearly three-quarters of the population. The virus moved eastward across the Pacific Ocean and took root in South and Central America, with the first cases appearing in late 2015; within months, more than two hundred residents in the continental U.S. who had not travelled contracted the infection locally. Last year, it seemed we were poised for a major outbreak. Then it stopped.
Despite the powerful technologies that scientists currently have to characterize pathogens and treat the infections they cause, the course and consequences of epidemics are still a source of surprise. Modern tools of molecular biology have enabled researchers to tear apart the Zika virus and decipher all of its genes and proteins, to map the antibodies and blood cells it mobilizes in infected individuals. But we still don’t know why some people contract the microbe with little or no illness, at most mild fever and muscle aches, while others suffer Guillain-Barré syndrome, a life-threatening paralysis. And we can’t distinguish between those pregnant women whose babies will be born deformed and others who seem to escape the most devastating neurological effects of the virus.
Further, there is no obvious reason for Zika’s rapid demise in the Americas. While spraying to eradicate mosquito-breeding sites may have contributed, it likely wasn’t sufficient to account for the precipitous decline in cases this year; the insects are simply too numerous and breed in too many areas. Nor would recommendations to refrain from unprotected sex be enough to blunt the virus’s spread. More possible is that much of the population in South America and the Caribbean was infected and became immune, with body defenses purging Zika from the blood and semen. This widespread “herd immunity” can help snuff out a disease in a particular geographic region, since the odds a mosquito picks up the virus plummet when most people bitten cannot carry the pathogen.
While the spectre of Zika in the Americas is fading, it’s wise to stay vigilant. Some experts worry that new cases of Zika recently reported in northern Mexico could presage another outbreak, with subsequent spread to U.S. border states. Relying on herd immunity is shortsighted, since over time fewer people will be infected and the virus can gain a new foothold. Indeed, the geographic distribution of Aedes aegypti, the mosquito species that transmits Zika, is expanding. The insect is infesting unexpected parts of North America and Europe; a population of A. aegyptiwas recently found in Washington, D.C., and appears to have survived four consecutive winters. The steady creep of climate change could bring A. aegypti farther north, where there is no herd immunity.
The definitive solution to prevent a return of the Zika epidemic is, of course, a broadly protective vaccine. Several efforts are ongoing, including a major trial sponsored by the National Institutes of Health. Ironically, the sharp drop in infections will make it difficult to demonstrate the efficacy of a candidate vaccine in field testing, since there is likely to be a relative dearth of new cases. Anticipating this obstacle, Anthony Fauci, who oversees the N.I.H. vaccine program, noted in Science that his team is poised to increase the number of participants in the trial, or to shift the location of testing to new hot spots.
But there is another concern about developing a Zika vaccine that has not received much attention. The virus does not exist alone in its ecosystem. The related pathogen dengue, which infects some four hundred million people annually and can cause excruciating skeletal pain and severe bleeding—breakbone fever—is carried by the same mosquitoes as Zika. Dengue has a boomerang biology: paradoxically, the antibodies produced after an infection can facilitate, rather than block, subsequent infections. The virus essentially uses the antibodies as Trojan horses, riding them covertly into our cells. In laboratory studies, Zika antibodies have appeared able to perform a similar deception, enhancing dengue’s capacity to attack white blood cells. An unintended consequence of a vaccine against Zika could be to elicit such antibodies and worsen dengue flareups. Although the W.H.O. declared, last November, that Zika is no longer a global emergency, we should not count on an end to outbreaks anytime soon—more likely, new beginnings.