Because the Zika virus is so new, until recently, scientists didn’t know how often infections in pregnant women led to microcephaly in their babies. Microcephaly is the birth defect that’s made the Zika virus international news; Zika otherwise typically causes only mild, flu-like symptoms. People with microcephaly have unusually small heads and often experience developmental delays.
The uncertainty about Zika’s risks for developing fetuses meant that a pregnant woman with Zika symptoms might be able to choose to have an abortion, depending on where she lived—but she would have to make that decision without looking to any numbers for guidance.
Now, scientists are finally getting at an answer: Fewer than one percent of women who become infected with Zika during their first trimester will have babies with microcephaly, according to a study published today in the Lancet. This figure is still up for debate, but even a preliminary risk percentage offers essential information to help pregnant women and their doctors decide what to do in the event of a Zika infection.
Though one percent may not seem like much, Zika is very contagious, and spreads easily between people via mosquitoes. That means even this low microcephaly risk can lead to many cases in areas where it’s difficult to avoid infection. Thirty-one countries and territories in Central and South America and the Caribbean have seen mosquito-carried Zika transmit between people within their borders, according to the World Health Organization’s latest situation report. Zika is expected to make its way to the southern United States with mosquito season, but Americans have a better chance of avoiding infection, thanks in part to air conditioning and strict mosquito control.
The authors of the Lancet study based their calculations on data from a now-finished outbreak of Zika virus in French Polynesia. Studies of pregnant women during Brazil’s current outbreak suggest microcephaly rates may be higher now than they were in French Polynesia, in 2013 and 2014.
One question that remains: whether just having a Zika infection is enough to give babies microcephaly, or if another trigger must simultaneously occur, such as a dengue fever infection.
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