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In 2015, a horror movie came to life. The mosquitoes that swarm almost all tropical climates began infecting people with a strange new virus. In most, Zika caused no symptoms, or a mild rash and fever. But if it happened to infect a pregnant woman, her baby could be born with severe birth defects. Zika dramatically increased the risk of a condition called microcephaly, or a clinically small head. Over the following years, about 4 to 9 percent of infected pregnant women gave birth to babies with permanent brain damage.
Suddenly, pregnant women in America and elsewhere were told not to travel to the Caribbean and South America. Expecting mothers in Miami, where local mosquitoes were transmitting the virus, stayed inside all summer long. Today, thousands of Brazilian families struggle to care for profoundly disabled 8-year-olds, “their limbs rigid, their mouths slack, many with foreheads that sloped sharply back above their dark eyes,” as The New York Times described in 2022.
Then, as quickly as it appeared, Zika vanished from global awareness. In 2016, most major news sites, including this one, largely stopped covering the disease regularly. Despite the absence of a treatment or vaccine, the world’s attention moved on.
There are good reasons for this: Zika cases dropped precipitously after 2016. And just a few years later, COVID ravaged the planet, giving us all something new to worry about. But that doesn’t mean Zika is gone. The disease is still out there, infecting people every day. There is still no Zika vaccine, and experts say another outbreak is likely before too long. In this way, Zika reflects a typical epidemic cycle—an emergent crisis, followed by a brief influx of resources, followed by rich countries’ long and fateful forgetting. “A lot of people have forgotten about Zika,” says Anna Durbin, a professor of global health at Johns Hopkins. “They think because we don’t see a big outbreak that it’s not there, but it’s definitely there. And it can be devastating for children born with congenital Zika syndrome.”
By 2017, Zika had burned through entire cities. Some experts estimate that the virus infected half the residents of Recife, a Northeastern Brazilian city and the outbreak’s epicenter. This swift onslaught was tragic, but it had an upside: Countries in the Caribbean and the Americas quickly achieved herd immunity, essentially starving the virus of new hosts. Cases fell off rapidly—in 2018, about 30,000 Zika cases were reported in the Americas, a region that spans between Argentina and Canada. Compare that with nearly 650,000 in 2016.
But despite this overall improved picture, the virus continues to circulate. In 2022, the Americas saw 40,528 cases of Zika. Brazil had the greatest number of cases that year, at more than 34,000, but Belize had the highest incidence per capita. As of early December, 31,780 cases were reported in the Americas in 2023. Microcephaly is far less prevalent, but it, too, is still occurring: Brazil saw 163 cases of Zika-linked microcephaly in 2022, according to the Pan American Health Organization, down from 2,033 in 2016. And growing evidence indicates that Zika can cause brain damage beyond microcephaly, including calcification in the brain and other, less noticeable issues. These effects are even less well tracked.
What’s more, these numbers are “probably just the tip of the iceberg,” says Albert Ko, a professor at the Yale School of Public Health. Up to 80 percent of people infected with Zika experience no symptoms and don’t get tested, and therefore would not be included in these case counts. There might be much more Zika circulating, and nobody is aware of it.
Tests for Zika are expensive and not readily obtainable, and the countries most affected by the virus have cash-strapped health systems. During the coronavirus pandemic, countries with limited lab resources shifted to diagnosing COVID, and Zika fell by the wayside. “The big problem is that many countries are not reporting Zika, or they’re not systematically testing for Zika,” Ko told me.
Herd immunity provided a temporary reprieve, but it also created a new problem. A lower incidence of Zika meant less commercial interest in making a vaccine, because the market for a Zika vaccine would by definition be smaller. Vaccine companies also struggled to find populations in which to test a vaccine, because too few people now had confirmed Zika infections. “The general momentum that was behind the development of a Zika vaccine ground to a halt,” says Jennifer Nuzzo, the director of the Pandemic Center at Brown University.
For now, this holding pattern might be acceptable, but it won’t be for long. Scientists don’t know how long Zika immunity lasts, and as time goes on, more and more people are being born who are immunologically naive: They’ve never been exposed to the virus before. Various experts predicted to me that five, 10, or 20 more years might pass without much Zika, after which we’ll see a smaller yet still sizable new outbreak.
When that happens, it will affect, primarily, poor women who live in the global South. In Recife, 97 percent of microcephaly cases occurred in children of women of low or medium socioeconomic status, according to Ernesto Marques, an infectious-disease professor at the University of Pittsburgh who has tracked Zika closely in Brazil. Poor women bore the brunt of Zika perhaps because they were more likely to live in areas without air-conditioning or good sanitation, and thus had greater exposure to mosquitoes. These women face both the ongoing stress of a potential Zika infection and the looming specter of another big flare-up in the community.
But the unresolved risk of Zika also threatens all citizens of these countries. Many nations most heavily affected by Zika rely on tourism to power their economy. Brazil is one of the largest economies in the world and attracts millions of visitors every year. Puerto Rico, which was hit hard by Zika, is part of the U.S., and thousands of women on the American mainland might travel back and forth to see family. Most experts I talked with said the risk was small for female travelers, because they would likely stay in hotels with air-conditioning and at least some mosquito control. But there’s no such thing as no risk.
Because Zika is currently circulating, but at low levels, official advice to pregnant women considering travel to these countries is ambiguous. Regarding almost every country in the Americas, the CDC warns that it has reported “past or current Zika virus transmission” but that “we do not have accurate information on the current level of risk.” It suggests that pregnant women and those who are considering getting pregnant “work with their health care providers to carefully consider the risks and possible consequences of travel to areas with risk of Zika.” In other words, caveat traveler.
When I asked the Pan American Health Organization, a spokesperson recommended that pregnant women who go to these places “take comprehensive measures to prevent mosquito bites, such as the use of insect repellents, the use of clothing (preferably light-colored) that covers most of the body, the use of bed nets and mosquito screens for windows and doors to prevent mosquitoes from entering the houses.” But who is going to wear a full-coverage outfit in a tropical climate? Asking every woman who is pregnant or even thinking about getting pregnant to apply these precautions consistently does not seem like a viable public-health strategy.
To be fair, some scientists are working to prepare for the next Zika wave; several met in the U.K. in December to discuss Zika research. Some organizations are working on a plan that would send mosquitoes infected with a bacterium that inhibits Zika into affected countries, in the hope that, over time, the infected mosquitoes would reduce Zika transmission. Several companies, including Moderna, maker of one of the COVID vaccines, are now working on a Zika vaccine. “But I think there’s still a lot of questions about what would the demand be? What is the target population? How are we going to fund that?” Durbin told me.
Experts I interviewed seemed frustrated that the world is not better prepared for when Zika strikes again. Multiple companies spun up a COVID vaccine in about a year because the U.S. government guaranteed that they’d get paid to do so. Zika emerged in Brazil eight years ago, and no similar guarantee seems forthcoming.
Instead, the political leaders of wealthy countries seem to jump from emergency to emergency, never quite internalizing the lessons from the most recent pandemic. Even before COVID hit, testing for Zika was difficult and spotty, including in rich countries like the U.S. But the American health-care system never learned from that failure: Testing for COVID was difficult and spotty in the early days of the coronavirus pandemic, and then, in 2022, testing for Mpox was difficult and spotty too.
It’s understandable that the U.S. government isn’t treating Zika as an emergency now that the crisis has subsided. But, as Nuzzo put it to me, “you don’t just shut down the fire station because you put the fire out.” If you do, you risk going up in flames.