In the past few weeks, scientists have published several of the most important—and really, only—studies of Zika infections in pregnant women. There was a study that found evidence of Zika virus in the brain of an aborted fetus that had microcephaly, a birth defect characterized by a small head and often severe disabilities. There was also a study that found Zika virus in amniotic fluid taken from two women who had Zika symptoms while they were pregnant, and later gave birth to babies with microcephaly. This week, the team behind the amniotic fluid research published a follow-up, where they did more careful genetic testing on the amniotic fluid.
There's one line you'll see over and over again in these studies (and subsequent news stories about them): They don't prove that Zika virus causes microcephaly. Although everything we know up to this point supports the idea that contracting Zika during pregnancy causes microcephaly in babies, scientists still can't say for sure whether that's true, or how it happens. Nevertheless, says Eric Rubin, an infectious disease researcher at Harvard University, "The evidence is pretty good, and we don't need absolute proof in order to act."
It was only recently that scientists suspected Zika had anything to do with microcephaly, after they noticed dramatic rises in the rates of both conditions in Brazil starting in the fall of 2015. The type of study required to prove causation, however, takes a lot of time, money, and study volunteers to complete. So far, scientists have only carefully analyzed a handful of cases of microcephaly in babies born in Brazil. "I don't think we're going to have a definitive answer in the near future," says Larry Kociolek, an infectious disease specialist at the Lurie Children's Hospital of Chicago.
"The evidence is pretty good, and we don't need absolute proof in order to act."
Meanwhile, as the infection continues to spread through South and Central America, it's likely mosquitos will start transmitting the virus in the United States by the spring. As a result, American public health officials need to tell people what to do. The level of evidence needed for scientists to be able to say "Zika virus causes birth defects" is totally different from the evidence doctors need to say "Zika is endemic here, so please wear insect repellent while you're pregnant and let's get a quick blood sample from you too." Thus, despite the scientific uncertainty, public health agencies around the world are issuing specific guidelines for what people should do if they live in or visit regions where the infection is endemic, especially if they or their partners are pregnant.
Even as more science emerges in the coming months, many of those guidelines probably won't change. As they are, they're pretty conservative. It helps that the suggestions, such as testing the blood of pregnant women who live in or visit Zika-endemic regions, carry generally low risks for mothers and babies.
There's another unfortunate consequence of the newness and uncertainty around Zika: Woman who are already pregnant and experiencing Zika-like symptoms have very little information with which to make a decision about whether to continue their pregnancies. (Of course, depending on the laws of the country in which they live, they may not always have much of a choice.) What's the likelihood the baby will have microcephaly? Does it matter at what stage of pregnancy the mother was infected? Does it matter how severe her Zika symptoms are? Is she still at risk if blood tests show she's infected, but she doesn't have any symptoms—a common outcome? Doctors don't know the answers to these vital questions.
In the future, better science about Zika may answer those questions and provide clues about how to better prevent additional cases of the birth defect.