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The Centers for Disease Control and Prevention said Wednesday the time has come to drop the squishy language previously used to describe the Zika virus. It isn’t just suspected of causing birth defects; it does cause birth defects.

Senior scientists involved in the agency’s Zika response argue in an article rushed to print by the New England Journal of Medicine that the virus has met a test used to determine whether an exposure — to a disease agent like a virus or a chemical, for instance — causes birth defects in a developing fetus.


The test is called the Shepard criteria, and it lays out seven factors that can be used to determine if an exposure causes birth defects. Zika currently meets four of the seven — and only three are needed to prove causality, the scientists said.

Scientists at the CDC and the World Health Organization have been clear for weeks that they believed infection during pregnancy was leading in some instances to babies with severe microcephaly — smaller than normal heads — and signs of profound brain damage.

But officially, they have avoided saying Zika infection “causes” brain-related birth defects, because in scientific terms there simply wasn’t enough evidence yet to say that definitively.


At the end of March, however, the WHO took the plunge, saying in a weekly Zika update that “based on observational, cohort and case-control studies there is strong scientific consensus that Zika virus is a cause” of microcephaly and other neurological disorders.

Wednesday’s article, however, is the first time scientists have laid out the argument for why Zika can be said to cause birth defects.

The lead author of the paper, Dr. Sonja Rasmussen, said it was important to go through that process, both to record the claim in the scientific literature, but also to persuade the public to take the threat seriously.

Rasmussen told STAT there continue to be those who doubt the link. CDC wants the public — especially pregnant women — to take the risk seriously, she said, and take the preventive steps it has recommended.

“We do these things because we want to be sure that people take action on the recommendations that we make,” said Rasmussen, editor in chief of CDC’s Morbidity and Mortality Weekly Report and an expert in birth defects research.

“We really want to do our very best to make sure that pregnant women hear the message that we’re giving them — that if you are thinking about traveling to one of these places where there is Zika virus transmission, please don’t. And if your partner has traveled to one of those places, to take precautions … to prevent sexual transmission.”

The Obama administration has been pressing Congress to approve an emergency request for additional funding to research Zika and support a public health response to the virus. Republicans have refused to date to approve the White House’s request for $1.9 million in funding, insisting that money committed but not yet used in the Ebola response be used first.

On Wednesday, there was a hint that Republicans may be getting ready to work toward providing additional money. The chairman of the House Appropriations Committee, Representative Hal Rogers of Kentucky, said during a committee hearing that his staff was working on an emergency funding bill that would provide more money in the current fiscal year, which ends in September. But he said his committee needs more information from the administration before it can produce a bill.

The administration has already announced, in the meantime, it would divert $500 million from the Ebola fund to the Zika response.

Dylan Scott in Washington contributed reporting.