For women living in or traveling to countries where Zika virus has taken hold, the most pressing question has been: If I get pregnant, and I get Zika, what are the chances that my baby will be born with microcephaly?
Until now, scientists had no idea. They are still trying to figure out exactly how Zika and microcephaly — the birth defect that can cause developmental delays, seizures, and problems with vision and hearing — might be linked.
A new study, published March 15 in The Lancet, calculated the risk of microcephaly during French Polynesia's outbreak from 2013-14, when 66% of the entire population got Zika.
And they finally found an answer to that pressing question. For pregnant women in French Polynesia who got Zika virus during their first trimester, the risk of their baby getting microcephaly was about one in 100. (In the US, where mosquitoes are not currently transmitting Zika, about 5.8 per 100,000 babies born have microcephaly.)
The good news, the authors say, is that this one-in-100 number is low compared to the risk of birth defects that can result from other maternal infections, like herpes or rubella.
This risk calculation is just a rough draft, though.
The researchers only studied the French Polynesian outbreak because it was over, so they had complete data to analyze. They caution against applying this risk calculation to Brazil and other areas of Latin America, where Zika is currently circulating.
It will take more studies to figure out the true risk of a baby getting microcephaly from a mother's Zika infection throughout the Americas. But at least now we have a starting point.
Dr. Laura Rodrigues, an infectious disease expert from the London School of Hygiene and Tropical Medicine, wrote in a comment published alongside the new study that she expected a higher risk number, based on the sheer number of microcephaly cases reported in Brazil.
This possible underestimation could be because the virus mutated to be more dangerous when it moved from French Polynesia to Brazil and the rest of Latin America. The mosquitoes that transmit Zika could also be slightly different from one area to the next. The environment and climate also differ, so they could play a factor in how the virus changed. There's also a possibility that in countries on high alert, microcephaly is being overreported.
Scientists aren't really sure why Zika became such a threat once it reached Brazil. Those are questions we still need answers to.
