Since Zika emerged as a threat to babies, it has been a mystery exactly how much of a danger the mosquito-borne virus poses to children.
But now, the largest study to follow kids who were exposed to the virus in the womb is providing more answers.
The study involved 1,450 babies who had been exposed to the virus, and who were 1-year-old by February 2018. Six percent were born with birth defects, and 14 percent developed problems that could be blamed on the virus by the time they turned 1, the study found.
"We're beginning to see the full spectrum of the impact of Zika," says Margaret Honein, director of the Division of Congenital and Developmental Disorders at the Centers for Disease Control and Prevention. The CDC released the study Tuesday.
"This is really our first look at how these children are doing as they grow and develop, and really emphasizes that the Zika story is not over, particularly for these children," Honein says.
Zika triggered an international public health emergency in 2016 when a large outbreak in Brazil revealed that the virus could cause babies to be born with very small heads and severely damaged brains when pregnant women get infected. The condition is called microcephaly.
It slowly has become more apparent that Zika-exposed babies could develop a range of other problems as well, including seizures, damaged vision and developmental disorders.
The CDC reported last year that about 5 percent of babies exposed in the womb are born with microcephaly and other birth defects. But the extent of the risk as children get older is just now starting to become clear.
The new analysis included babies born in U.S. territories such as Puerto Rico and in U.S. freely associated states, such as the Marshall Islands. It found that the risk for birth defects including microcephaly and vision damage is slightly higher — about 6 percent. And 1 in 7 — 14 percent — developed some kind of problem that could have been caused by the virus by their first birthday.
For example, 20 babies in the new analysis whose heads were normal at birth had microcephaly by the time they turned 1.
"That happened because their brain was not growing and developing properly," Honein says.
Babies also developed complications including cognitive problems, difficulties walking, moving and swallowing, and seizures.
"It's really important that parents and doctors work together to make sure children get all the evaluations they need, even if they look healthy when they are born," Honein says.
For example, only about one-third of the Zika-exposed babies in the study had an eye exam by an eye specialist.
It's also important to continue to follow these children, she says.
"We are still in the early stages of learning about Zika. So we don't yet know what sort of problems might emerge when the children are 2 years old or 3 years old or when they reach school age," Honein says.
There are no major Zika outbreaks occurring right now. But Honein stresses Zika is still being transmitted in many countries and outbreaks still could occur.
So pregnant women and couples trying to conceive should continue to protect themselves while living or visiting places where Zika is being transmitted. The virus is primarily spread by mosquitoes, but can also be spread sexually.
The CDC on Tuesday also issued new interim guidance for men who were exposed to the virus. The agency is now recommending these men wait three months after exposure before trying to conceive. The CDC had previously recommended waiting six months. But the latest science suggests the virus doesn't remain infectious in semen as long as previously thought.
ARI SHAPIRO, HOST:
Zika may have disappeared from the headlines, but the mosquito-borne virus is still lurking around the world. And there's troubling news out today about the threat Zika poses to babies in their first year of life. NPR health correspondent Rob Stein has the details.
ROB STEIN, BYLINE: Zika triggered an international public health emergency two years ago when a big outbreak in Brazil revealed the virus could devastate babies' brains. The crisis has passed, but the virus is still spreading in nearly 100 countries. So Peggy Honein at the CDC says it's crucial to get a better understanding of what happens to babies when they're exposed to Zika in the womb.
PEGGY HONEIN: And this is really our first look at how these children are doing as they grow and develop and really emphasizes that the Zika story is not over, particularly for these children.
STEIN: The CDC studied more than 1,400 Zika-exposed babies who doctors follow at least through their first birthdays.
HONEIN: This is the largest study of children reaching 1 year old and beginning to see the full spectrum of the impact of Zika.
STEIN: Six percent were born with birth defects like very small heads and badly damaged brains, a condition known as microcephaly. But by the time these kids turned one, 14 percent had developed some kind of problem that could have been caused by Zika.
HONEIN: There were 20 babies that had a normal head circumference when they were born, and their measurements were normal. But as they grew, they developed microcephaly. And that happened because their brain was not growing and developing properly.
STEIN: And that's not all.
HONEIN: We're seeing vision problems, some hearing loss. We're seeing motor difficulties, moving their hands to pick up objects around them, cognitive problems, seizures, swallowing difficulties.
STEIN: And Honein says, who knows what will happen to these kids as they grow older?
HONEIN: We are still in the early stages of learning about Zika, so we don't yet know what sort of problems might emerge when the children are 2 years old or 3 years old or when they reach school age and what sort of assistance they're going to need to reach their full potential.
STEIN: So Honein says these kids have to be monitored closely to be on the lookout for problems.
HONEIN: It's critically important that parents and doctors work together to make sure children that have possible Zika exposure during pregnancy are getting all the care and evaluation they need even if they look healthy when they're born.
STEIN: And the CDC is following thousands more babies to continue to get a better idea of just how dangerous Zika is. Rob Stein, NPR News. Transcript provided by NPR, Copyright NPR.