Zika virus burned through the Americas in 2015 and 2016, dominating headlines with alarming reports of birth defects, then seemed to vanish almost overnight. The virus didn’t disappear, but transmission dropped to low levels starting in 2018 and has stayed there since. No locally acquired cases have been reported in the continental United States since 2017, and as of 2025, the CDC has no active Zika travel health notices for any country in the world.
So what actually happened, and should you still be concerned? The story involves herd immunity, a stalled vaccine effort, and a virus that experts say will almost certainly come back.
Why the Epidemic Burned Out
When Zika arrived in the Americas around 2015, almost no one in the Western Hemisphere had prior immunity. The virus spread explosively through populations that were completely susceptible, carried by the same species of mosquito responsible for dengue and yellow fever. By the end of 2016, millions of people across Latin America and the Caribbean had been infected.
That rapid spread was also the epidemic’s undoing. As vast portions of the population became immune, the virus ran out of new hosts. This herd immunity reduced the number of susceptible people to the point where the virus could no longer efficiently cycle between humans and mosquitoes, and case counts plunged. Infectious disease experts had predicted this pattern because other mosquito-borne viruses in the same family behave the same way: they tear through a population, then fade into the background.
Better mosquito control didn’t drive the decline. Climate factors weren’t responsible either. The primary explanation is simply that the virus infected so many people so quickly that it exhausted its pool of potential hosts. In the United States specifically, the drop also reflected fewer infected travelers arriving from Latin America and the Caribbean, which meant local mosquitoes had fewer opportunities to pick up and spread the virus. The last locally acquired cases in the continental U.S. occurred in Florida and Texas in 2016 and 2017.
Where Zika Still Circulates
Zika hasn’t been eradicated. The WHO reports that transmission persists in several countries at low levels, a pattern that has held from 2018 through at least the end of 2023. The virus continues to circulate quietly in tropical regions where its mosquito vector lives, but without the explosive outbreaks that defined the 2015-2016 period.
The CDC currently lists no countries under an active Zika travel health notice, though it maintains a broader list of countries and territories with “current or past” transmission where some risk may exist. The practical reality is that for most travelers, Zika is no longer a front-of-mind concern the way it was a decade ago. That said, the CDC still recommends testing if you develop Zika symptoms after traveling to an area with known transmission history, and pregnant travelers warrant extra caution. Testing is no longer routinely recommended for pregnant women without symptoms, though it can still be considered after travel to affected areas.
Why There’s Still No Vaccine
During the height of the crisis, researchers mobilized quickly. Multiple vaccine candidates entered clinical trials, and by 2024, at least 16 vaccines had reached phase 1 or phase 2 testing. Several promising approaches were explored: inactivated virus vaccines, mRNA-based candidates (including ones developed by Moderna), DNA vaccines, and vaccines using modified viruses as delivery vehicles.
Three candidates made it to phase 2 trials, but none succeeded. One was withdrawn for business reasons. Another lost funding in mid-2024. The third, developed by the National Institute of Allergy and Infectious Diseases, was completed early without ever being able to assess whether the vaccine actually worked, because so few people at the trial sites were getting infected with Zika. That last detail captures the core problem: the epidemic ended before scientists could prove a vaccine works.
Demonstrating that a vaccine prevents birth defects requires large numbers of people being exposed to the virus during a trial. With transmission at a trickle, that’s essentially impossible. Investment in lab infrastructure, disease surveillance, and clinical trial site preparation also dried up as case counts fell. The result is a frustrating catch-22: the vaccine is most needed during an outbreak, but it can only be tested and approved during an outbreak, and by the time one arrives, it may be too late to deploy a vaccine that hasn’t yet been licensed.
The Risk of a Comeback
The immunity that shut down the 2016 epidemic won’t last forever. New children are born without immunity every year. Over time, the proportion of susceptible people in affected regions will grow, and at some point the virus will have enough potential hosts to spark another large outbreak. Experts have described this as a question of when, not if.
Climate change adds another layer of risk. The mosquito species that carries Zika thrives in hot, humid conditions, and its geographic range has expanded significantly over recent decades as global temperatures have risen. Warmer weather does several things that favor transmission: mosquitoes reproduce faster, feed more frequently, live longer while infectious, and colonize areas that were previously too cool. The 2015-2016 epidemic itself followed an unusually hot and rainy El Niño season with severe flooding, conditions that climate models predict will become more common.
The combination of waning population immunity and expanding mosquito habitat means the conditions for a future Zika resurgence are gradually building. The unpredictability of when and where that resurgence might happen is precisely what makes it so difficult to prepare for, whether through vaccine development, mosquito control, or public health planning.
What This Means for You Now
If you’re traveling to tropical regions, Zika is currently a low but nonzero risk. Standard mosquito bite prevention still applies: insect repellent, long sleeves in high-risk areas, and staying in places with screens or air conditioning. If you’re pregnant or planning to become pregnant, it’s worth checking the CDC’s country-specific guidance before traveling to tropical destinations, even in the absence of active travel notices.
Zika symptoms, when they occur, are typically mild: fever, rash, joint pain, and red eyes lasting a few days to a week. Most infected people never develop symptoms at all. The serious danger was always to developing fetuses, where infection during pregnancy can cause microcephaly and other severe brain abnormalities collectively known as congenital Zika syndrome. That risk hasn’t changed. What’s changed is the probability of encountering the virus, which for now remains low.