Researchers hone in on risk of Zika birth defect

NEW YORK -- As the international epidemic of Zika virus disease has unfolded and led to devastating birth defects for at least 1,300 children in eight countries, a question has persisted: What is the chance an infected pregnant woman will have a baby with these defects?

Researchers don't have a complete answer, but they are honing in on one.

The largest study to look at the question states the risk of one especially severe type of birth defect is "substantial" -- in the range of 1 percent to 14 percent.

It also reinforces the understanding women infected in the early stages of pregnancy face the greatest risk.

The range is wide because researchers are relying on imprecise and incomplete information as they to try to estimate the level of risk in advance of what they say is likely transmission of Zika by mosquitoes in the U.S. later this year.

The study focused on what was seen in just one place, northeast Brazil. It looks only at microcephaly, a condition in which a baby's skull is much smaller than expected because the brain hasn't developed properly. But health officials said Zika can cause other birth defects, too.

"These numbers are probably only the tip of the iceberg," said Dr. Neil Silverman, a UCLA professor of obstetrics who has been advising the California Department of Public Health on Zika issues.

A study in Rio de Janiero published earlier this year suggested there is a nearly 30 percent risk for all kinds of Zika-related birth defects and fetal death. The new research doesn't contradict that, said Silverman, who was not involved in the latest research.

The new study was done by government scientists at the Centers for Disease Control and Prevention. It was published online Wednesday by the New England Journal of Medicine.

The Zika virus causes only a mild and brief illness in most people. But in the last year, infections in pregnant women have been linked to fetal deaths and to potentially severe birth defects, mostly in Brazil. Last month, the CDC said there was enough evidence to declare Zika causes microcephaly and other brain defects.

The virus is spread mainly through the bite of a tropical mosquito called Aedes aegypti. The bug can be found in the southern United States, but there's no evidence they've spread the virus in the U.S.

The new study is based on about 400 babies with microcephaly in the Brazilian state of Bahia, diagnosed between July and February.

The researchers made estimates based on what data they could get but faced challenges. They weren't certain all 400 microcephaly cases were correctly diagnosed. They didn't know for sure how many of the mothers of the affected children had a Zika infection, nor how many pregnant women in Bahia in total were infected with Zika virus during that time period.

But based on other data from Bahia and from two earlier Zika outbreaks, they could make calculations that accounted for varying scenarios.

A large Zika epidemic would lead to more babies born with birth defects, but the chance an individual baby would be affected shrinks as an outbreak progresses. That's because a large outbreak would expose more women to the virus and allow them to develop immunity.