On Oct. 18, 2019, in the New England Journal of Medicine, researchers published a comprehensive review of this Zika literature to date, and noted that while the rate of microcephaly is not as high as what was once predicted, the virus can have long-lasting consequences for those exposed to it in utero. They also warned that Zika could and will likely strike again.

Since the Zika epidemic of 2015 and 2016 sickened thousands of people in the Americas, and resulted in 3,700 babies born with birth defects —including microcephaly —researchers have produced a deluge of academic papers in an effort to answer key questions about the flavivirus.

Zika is asymptomatic in 50% to 80% of cases, but, unlike other mosquito-borne viruses, it can also cause birth defects when pregnant women are infected. This feature of the Zika epidemic caused the World Health Organization (WHO) to declare a public health emergency of international concern in 2015, when the first babies from northeastern Brazil with microcephaly were born.

At first, estimates for birth defects produced by the Brazilian ministry of health were much higher than what is now understood to be a 4% to 6% risk of microcephaly in a baby born to a mother with Zika.

Presently, Zika transmission has slowed and ceased throughout much of the epidemic regions. In 2018, there were fewer than 30,000 cases reported, compared with more than 500,000 cases reported at the peak of the pandemic in 2016, the authors write.

In general, only 20% to 30% of babies born to a mother infected with Zika will contract the virus, and 5% to 14% of those babies will have congenital Zika syndrome, including cognitive, neurological, and vision problems. In a study on US babies born to mothers with Zika, researchers found that 9% of the babies who appeared healthy at birth later developed at least one neurodevelopmental abnormality before they reached the age of 2.

CIDRAP has partnered with the University of Texas Medical Branch at Galveston to work on a WHO research and development roadmap for Zika, which includes work on a Zika vaccine.