Rio de Janeiro, Brazil – Five years after the World Health Organization declared Zika a global public health emergency, a landmark study has uncovered that the virus’s impact extends far beyond the microcephaly crisis, silently impairing brain development in a significant number of children who appeared healthy at birth. Researchers from the Oswaldo Cruz Foundation and the University of California, Los Angeles, published findings today in Nature Medicine revealing that nearly one in three children exposed to Zika in the womb—but born with normal head circumference—later show deficits in cognitive function, motor skills, and language development.
“The initial narrative focused on the most severe outcome, microcephaly, but what we are seeing now is a much broader, subtler second wave of damage,” said Dr. Karin Nielsen-Saines, a pediatric infectious disease specialist at UCLA and lead author of the study. “These children grew into their disabilities. For many families, it was a delayed diagnosis, and for clinicians, a missed window for early intervention.”
The study followed 216 children in Rio de Janeiro from birth to age four, administering standardized neurodevelopmental assessments. Of the 131 children who had normal head size at delivery, 34% scored below the cutoff for average performance in two or more developmental domains. Common deficits included difficulty with fine motor tasks like stacking blocks, delayed speech milestones, and poor visual-spatial reasoning. The findings were compared against a control group of unexposed children from the same socioeconomic background, where only 12% showed similar delays.
A ‘Shadow Cohort’ of Hidden Cases
During the 2015-2016 epidemic, Brazil reported more than 3,700 confirmed cases of congenital Zika syndrome, defined by microcephaly and other birth defects. However, new modeling suggests that the total number of children with neurological complications—including those without microcephaly—could exceed 100,000 across Latin America and the Caribbean. Many of these children remain undiagnosed.
“We have a shadow cohort walking into primary schools,” said Dr. Celina Turchi, an epidemiologist at the Oswaldo Cruz Foundation and co-author of the study. “They may not look different from their peers, but they struggle to follow instructions, to hold a pencil, or to play with others. Without screening, they are labeled as ‘slow learners’ when the root cause is a known viral infection.”
The study used gold-standard neuropsychological tests, including the Bayley Scales of Infant Development, to track children whose mothers confirmed Zika infection via PCR testing during pregnancy. The team controlled for variables such as maternal education, household income, and head circumference at birth, isolating Zika exposure as the primary risk factor for impaired outcomes.
Implications for Public Health Systems
These findings place urgent pressure on health systems already strained by COVID-19. In Brazil, waiting lists for pediatric neurologists and speech therapists run as long as two years. Many of the affected children are now reaching school age, where deficits become more apparent—and harder to remediate.
“The cost of late intervention is enormous,” said Dr. Albert Ko, an epidemiologist at Yale School of Public Health who was not involved in the study. “We know that early childhood development programs—physical therapy, language therapy, caregiver coaching—can dramatically improve outcomes. But they only work if the child is identified early.”
The study’s authors recommend that all children with documented in utero Zika exposure receive scheduled neurodevelopmental screenings at 12, 24, and 36 months, regardless of head circumference at birth. They also call for long-term longitudinal studies to follow these children into adolescence, as emerging evidence suggests that some neurological deficits may not become apparent until higher-order thinking skills, such as planning and impulse control, are required.
The Human Toll: One Family’s Journey
In the Vila Kennedy favela, Maria dos Santos described watching her daughter, Beatriz, struggle to tie her shoelaces at age seven—a task her younger, unexposed brother mastered at age five. Beatriz was born with a normal head circumference. Her Zika diagnosis came only after the research team enrolled her in the study.
“The doctors said she was fine. I thought I was a bad mother for not being able to teach her,” said dos Santos. “Now I know. It was a virus. I just wish I had known two years earlier.”
This article was reported in collaboration with the Oswaldo Cruz Foundation and the UCLA David Geffen School of Medicine. For more information, visit the WHO’s Zika virus fact sheet or the CDC’s resources on congenital Zika syndrome.
