January 08, 2026
8 min read
The Zika virus epidemic that began in Brazil in 2015 and quickly spread to other countries caused an unprecedented flood of neurological disease, especially among infants, and changed the perception of the mosquito-borne disease forever.
A virus once believed to be largely benign was suddenly ravaging the brains of infants and causing central nervous disease in adults, leading WHO to declare an international public health emergency in February 2016, which lasted 9 months.
Image: Children’s National Hospital
Ten years later, many questions remain, including why the virus became so harmful and whether it will cause another widespread epidemic. Scientists continue to pursue vaccines and therapeutics for Zika while warning that the virus is still circulating at lower levels in some areas, and causing tens of thousands of cases a year in Brazil.
We checked in with experts to ask what has been learned over the last 10 years, what science still does not know about the virus, and what is in the pipeline for Zika prevention that could potentially stave off another epidemic.
A decline in cases
Overall, rates of Zika have remained low globally since the epidemic, with ongoing cases occurring primarily in Latin America and the Caribbean and occasional travel-associated infections popping up in other countries, noted Omar S. Akbari, PhD, a biologist, mosquito expert and Tata Chancellor’s Endowed Professor in Cell and Developmental Biology at the University of California, San Diego.
“Major epidemic-scale outbreaks are not occurring at present,” he told Healio | Infectious Disease News.
Brazil has seen significantly more cases than any other country. According to the Pan American Health Organization (PAHO), Brazil recorded a total of more than 300,000 cases in 2015 and 2016. Since then, cases have decreased but remain high, totaling more than 43,000 in 2024 and more than 24,000 in 2025.
Elsewhere, Argentina and Bolivia also reported more than 1,000 cases last year.
Experts said there are likely several factors contributing to the higher burden of Zika in Brazil. In northeast Brazil, for example, exposure to Zika-spreading mosquitoes is high, and immunity to the virus among newborns is variable — two factors that could amplify transmission when combined, Akbari said.
Additional factors include a tropical climate, crowded communities and poor sanitation, according to Kristina M. Adams Waldorf, MD, a professor of obstetrics and gynecology and adjunct professor of global health at the University of Washington, who noted that these factors only partially explain why Brazil has so many more cases than any other country.
The United States has seen a handful of travel-related cases in recent years, but no locally acquired Zika virus infections since 2017, according to CDC data. During 2016 and 2017, however, there were more than 5,300 travel-associated cases in U.S. states, widespread transmission in territories including Puerto Rico and the U.S. Virgin Islands numbering more than 37,000 cases, as well as local transmission for the first time in Florida and Texas.
Locally acquired cases of Zika virus continue to be reported in the dozens in U.S. territories, although the CDC notes that the current method of diagnosis — antibody testing — is unable to distinguish between recent or past infection.
Sarah B. Mulkey, MD, PhD, a fetal and neonatal neurologist at Children’s National Hospital in Washington, D.C., said the reported case rates suggest low levels of Zika virus circulation, but she acknowledged that the counts are likely not accurate.
“People can have asymptomatic infections, and testing can be positive for other flavivirus like dengue when the infection may be Zika,” she told Healio | Infectious Disease News.
Congenital Zika syndrome
The public health emergency declared by WHO in 2016 was in response to the explosion of microcephaly and other neurological conditions linked to Zika, especially among babies born from mothers who were infected during their pregnancies.
A large study by Oliver J. Brady, DPhil, and colleagues demonstrated that women infected with Zika virus during early pregnancy were 17 times more likely to have a child with microcephaly, according to results published in 2019.
The study identified between 3.6 million and 5.4 million cases in Brazil from January 2015 to May 2017. And although there are other potential causes of microcephaly in Brazil — including other mosquito-borne diseases — Brady and colleagues said they found no evidence that anything other than Zika was responsible for the surge in cases during the study period.
“We have learned a lot about how Zika virus damages the developing brain, and the reliable markers that indicate a severe case, which we call congenital Zika syndrome,” Adams Waldorf said.
According to the CDC, conditions that occur in congenital Zika syndrome include severe microcephaly, brain development problems, difficulty swallowing, hearing and vision damage, seizures, joints with limited range of motion and stiff muscles.
A study by Karin Nielsen-Saines, MD, MPH, and colleagues showed that around one-third of children aged younger than 3 years who were exposed to Zika virus in utero experienced below-average neurodevelopment and/or eye abnormality or hearing assessments.
Questions remain about the long-term consequences of exposure to Zika virus in utero, especially among children without birth defects or microcephaly.
“Children born during the Zika virus epidemic are now school age, which is a time of rapidly advancing higher order cognitive skills,” Mulkey said. “Since children born during the epidemic are only now at the age when these skills can be tested, we do not yet know whether the children with prenatal Zika virus exposure who are normocephalic will have impacts on cognitive skills such as memory, attention and executive functioning.”
Data have begun showing that children who do not have obvious birth defects or microcephaly can still be at risk for a spectrum of neurodevelopmental effects, including impacts to cognitive skills.
Mulkey and colleagues assessed dozens of Columbian children whose mothers had Zika during pregnancy but who did not develop microcephaly and compared them with children from the same community who were not exposed to the virus. Overall, they found that the Zika-exposed cohort had lower IQ scores by the time they entered school compared with the non-exposed controls, Mulkey noted. The results aligned with previous findings that showed children exposed to Zika in utero demonstrated developmental differences into preschool age, even in the absence of congenital Zika syndrome.
“We have learned that children with … congenital Zika syndrome have an increased risk of mortality due to the severity of their neurological disabilities. We have also learned that children with in utero exposure to Zika virus who do not have birth defects or microcephaly can still be at risk for a spectrum of neurodevelopmental effects including impacts to areas of cognitive skills,” Mulkey said.
She said that understanding the long-term effects that congenital Zika infection has on the brain can help prepare for future epidemics because the long-term outcomes are likely also applicable to other prenatal viral exposures.
Zika prevention
The best way to prevent fetal brain injuries from Zika virus infection is to prevent the infection, Adams Waldorf noted.
Although there are no vaccines available for the prevention of Zika, it remains an active area of research.
A recent review published in The Lancet Infectious Diseases identified 16 vaccines in phase 1 or phase 2 trials. Results from some of the phase 1 trials assessing inactivated, live-attenuated, mRNA, DNA, virus-vectored and recombinant protein vaccine showed that the approaches are “generally well tolerated and immunogenic,” and three phase 2 trials of inactivated, mRNA and DNA vaccines were “not completed as planned.”
Because there is no large ongoing Zika virus outbreak, researchers cannot test vaccine candidates in phase 3 field trials. The lack of cases means the only way to test vaccine candidates is to develop safe human challenge trials, in which participants are deliberately infected with Zika. Several such trials have been completed or are underway.
There are other obstacles that could get in the way of carrying a Zika virus vaccine across the finish line, according to Adams Waldorf.
“Many Zika virus vaccine candidates exist and could be tested if there is a surge in transmission to determine whether they work,” she said. “However, the current administration is planning changes to how vaccines are approved in this country, which would make it difficult — if not impossible — to get a new vaccine approved.”
Other tools besides vaccines can and are being deployed for Zika prevention. Akbari listed several techniques focused on the mosquitoes themselves that could be beneficial, such as releasing Wolbachia-infected mosquitoes into the wild, which has been approved by the EPA for mosquito population suppression since 2017.
A bacterium, Wolbachia is found in up to 60% of insects, but not usually Aedes mosquitoes, the primary vector of Zika in the U.S.
Male Aedes mosquitoes infected with Wolbachia in a lab and then released into the wild mate with wild female mosquitoes, producing eggs that will not hatch, decreasing the population. Studies have shown that it can be an effective way to control Aedes mosquitoes.
Akbari mentioned several other newer methods of mosquito control that could “proactively suppress” populations of Zika-spreading mosquitoes, including precision guided sterile insect release and genetically modifying mosquitoes to be “self-limiting” — or unable to survive in the wild.
A 2024 study by Pattamaporn Kittayapong, PhD, and colleagues assessed the impact of open field release of sterile male mosquitoes in an eastern Thailand village and found that, after 6 months, there was a significant reduction in the average egg hatch rate (84%) and average number of female mosquitoes per household (97.3%).
Is another epidemic inevitable?
Although Zika transmission has subsided in the U.S., Mulkey noted that there have been outbreaks of chikungunya — a flavivirus similar to Zika — and a rare locally reported case of chikungunya in New York in a person who had not traveled recently.
“If we see chikungunya in the U.S., we may also see Zika and dengue,” she said. (All three are spread by Aedes mosquitoes.)
Should an outbreak of Zika or a similar virus occur, the CDC has a response plan in place that is based on information learned during the 2015-2016 epidemic, an agency spokesperson told Healio | Infectious Disease News. The spokesperson said annual funds provided to state and local municipalities support surveillance, diagnostics and response to vector-borne diseases such as Zika.
“More than anything, I wish we knew when Zika virus would roar back as a new epidemic,” Adams Waldorf said.
There are factors that can indicate a possible resurgence.
“When a new population of nonimmune individuals [grows] within a community that has active Zika virus spread, a new epidemic may occur. Usually, this happens about 10 to 20 years after the first epidemic,” Adams Waldorf said. “We are now at a point that we warned about in 2017 in which an epidemic might occur.”
Improvements to mosquito control and the residual immunity that was gained during the Zika epidemic suggest that a repeat “is not inevitable,” Akbari said.
“Vigilance and readiness are critical,” he said. – by Caitlyn Stulpin
For more information:
Kristina M. Adams Waldorf, MD, can be reached at adamsk@uw.edu.
Omar S. Akbari, PhD, can be reached at oakbari@ucsd.edu.
Sarah B. Mulkey, MD, PhD, can be reached at sbmulkey@childrensnational.org.
Sources/Disclosures
Source:
Healio Interviews
References:
- Brady OJ, et al. PLoS Med. 2019;doi:10.1371/journal.pmed.1002755.
- CDC. Mosquitoes with Wolbachia. https://www.cdc.gov/mosquitoes/mosquito-control/mosquitoes-with-wolbachia.html. Updated May 14, 2024. Accessed Dec. 12, 2025.
- CDC. Potential range of Aedes mosquitoes. https://www.cdc.gov/mosquitoes/php/toolkit/potential-range-of-aedes.html. Updated July 1, 2024. Accessed Jan. 3, 2026.
- CDC. Zika Cases in the United States. https://www.cdc.gov/zika/zika-cases-us/index.html. Updated Dec. 16, 2025. Accessed Jan. 3, 2026.
- Kittayapong P, et al. PLoS Negl Trop Dis. 2019;doi:10.1371/journal.pntd.0007771.
- Mulkey SB, et al. Pediatr Res. 2025;doi:10.1038/s41390-025-03981-7.
- Nielsen-Saines K, et al. Nat Med. 2019;doi:10.1038/s41591-019-0496-1.
- Ostrowsky JT, et al. Lancet Infect Dis. 2025;doi:10.1016/S1473-3099(24)00750-3.
- PAHO. Zika: Analysis by country. https://www.paho.org/en/arbo-portal/zika-data-and-analysis/zika-analysis-country. Accessed Jan. 3, 2026.
- WHO. Zika virus. https://www.who.int/news-room/fact-sheets/detail/zika-virus. Updated Nov. 6, 2025. Accessed Dec. 8, 2025.
Disclosures:
Adams Waldorf and Akbari report no relevant financial disclosures. Mulkey reports receiving research support from the NIH for a study on childhood outcomes after antenatal Zika virus exposure and having a contract with the CDC for technical expertise relating to congenital infections.
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