After months of hesitation, U.S. health officials now say that the Zika virus is indeed the cause of severe brain damage in the infants of some women who were infected with the virus during pregnancy.
A CDC review published online Wednesday in the New England Journal of Medicine describes evidence of what U.S. health officials now call a causal relationship between the virus and a severe form of microcephaly and intracranial calcifications.
The study marks a turning point in the current Zika outbreak, says Dr. Tom Frieden, chief of the CDC. "There's still a lot that we don't know, he says. "But there is no longer any doubt that Zika causes microcephaly."
A causal link between the bite of a mosquito and brain malformations is unprecedented, he says. "And it is because this was so unprecedented that we have until now waited to say that we have concluded that there is a causal link."
There was no single smoking gun, CDC officials say. Dr. Sonja Rasmussen, editor-in-chief of CDC's Morbidity and Mortality Weekly Report and an author on the report, says the conclusion came from looking systematically through the accumulating evidence.
The team used two approaches to confirm the link. The first method involved finding an overlap between a rare exposure and a rare defect. It was the same approach used years ago to confirm the effects of rubella on a pregnancy, and the same used to characterize fetal alcohol syndrome.
The second method looked at the timing of outbreaks in Brazil and French Polynesia and subsequent increases in birth defects. Researchers also looked at rare case reports of pregnant women who traveled to places with Zika virus transmission and subsequently gave birth to babies with microcephaly.
There were other clues. These babies have an unusual and particularly severe form of microcephaly. Plus, the virus has been found in the brain tissue of some babies with microcephaly who died.
Still, a lot of questions remain, the health officials say.
For example, it's unclear whether the baby's risk of brain damage is greater if the mother has symptoms of a viral infection while pregnant, compared with an infected woman who never experiences symptoms. And it's unclear how much that risk changes if a woman is infected early in pregnancy versus late in pregnancy.
Among the most pressing remaining questions, Rasmussen says, are these: "Does Zika cause defects other than those of the brain? Does Zika cause learning problems later in life? Are pregnancy losses, miscarriage, stillbirth among some women infected by Zika virus the result of the infection? And finally, are there other factors involved? For example, has another infection occurred at the same time as the Zika infection that might affect the risk of birth defects?"
Frieden says it could take years to get all those answers.
ROBERT SIEGEL, HOST:
There is no longer any doubt the Zika virus causes devastating birth defects. That's the conclusion announced today by the Centers for Disease Control and Prevention. NPR health correspondent Rob Stein joins us now with the details. Rob, we've been hearing for a long time that Zika is a threat to pregnant women and to their developing fetuses. What exactly is new here?
ROB STEIN, BYLINE: Yeah, so Robert, you're right. Health authorities have been saying for quite some time that there was mounting evidence that Zika causes microcephaly. That's the condition which babies are born with usually small heads and severe brain damage. But what they had been saying is that it hadn't been proven. And what the CDC said today is that there's no doubt - Zica does cause microcephaly.
SIEGEL: And what are they basing that conclusion on, what kind of research?
STEIN: It's not based on any single piece of single new research. Instead, scientists reviewed all the scientific evidence that's emerged since Zika kind of exploded in Latin American and the Caribbean and babies started being born with microcephaly in Brazil. And they used that evidence to analyze whether it was corresponding to two sets of criteria that are widely accepted in the scientific community when you're trying to determine whether something cause birth defects. For example, whatever the culprit is - the suspected culprit - exposure has to occur at a very specific time in a pregnancy. And the CDC says it's clear that when it comes to Zika and microcephaly, women who are exposed in their first trimester or early in their second trimester are at greatest risk for microcephaly.
SIEGEL: Well, does this conclusion of the CDC's - does it change what people should do to protect themselves?
STEIN: You know, in a lot of ways, it is just crossing the T's and dotting the I's. The CDC's not changing its, like, travel recommendations or anything like that. But nevertheless, the CDC is saying that it should be kind of a turning point in the fight against Zika and microcephaly. And they're hoping it'll be kind of a wake-up call for lots of reasons. Here's Tom Frieden - he's the head of the CDC - explaining one of them.
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TOM FRIEDEN: This is an unprecedented association. Never before in history has there been a situation where a bite from a mosquito could result in a devastating malformation.
STEIN: So Frieden is saying that it's more important than ever that women follow the guidelines and the recommendations that the agency's been issuing for how they can protect themselves from Zika. For example, if you're a pregnant woman, you should not travel to places where the virus is spreading. And Frieden's also hoping that this will kind of prompt public health authorities to kind of double down on their fight to stop the spread of the virus.
SIEGEL: So the CDC says we now know something more about the Zika virus. So there's still a lot we don't know about it though.
STEIN: Oh, absolutely. I mean, what they're saying is OK, so Zika causes microcephaly, but it doesn't always cause microcephaly. Not every woman who gets infected has a baby with these severe birth defects. And the big question is how often does it do that? Is it 1 percent of the time? Is it 30 percent of the time? They really don't know. And another big question is is Zika causing other things? There's mounting evidence that microcephaly and the severe brain damage might be kind of just the tip of the iceberg, that Zika might be causing other problems, like miscarriages or babies to be born with less-severe brain damage or maybe complications like deafness or blindness. We really don't know if that's the case or how often that's the case.
SIEGEL: OK, that's NPR's Rob Stein. Rob, thanks.
STEIN: Oh, sure, nice to be here. Transcript provided by NPR, Copyright NPR.