CDC Confirms Link Between Zika Virus, Microcephaly

Zika microcephaly
Zika microcephaly
After a rigorous review of evidence, the CDC has confirmed that Zika virus causes microcephaly and other fetal brain abnormalities.

After a period of caution, the Centers for Disease Control and Prevention have confirmed that Zika virus does indeed cause microcephaly and other fetal brain defects.

“This study marks a turning point in the Zika outbreak,” CDC director Tom Frieden, MD, MPH, said in a statement. “It is now clear that the virus causes microcephaly.  We are also launching further studies to determine whether children who have microcephaly born to mothers infected by the Zika virus is the tip of the iceberg of what we could see in damaging effects on the brain and other developmental problems.”

The confirmation of the link between Zika and microcephaly did not come from one specific piece of evidence, but instead a body of mounting data that indicated increased prevalence of microcephaly during  Zika outbreaks in Brazil and French Polynesia, identification of a rare phenotype in infected fetuses or infants, and evidence of Zika infection in fetal brain tissue.

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Declared a global public health emergency by the World Health Organization in February, Zika virus has spread rapidly throughout the Americas since its identification in Brazil in 2015. An increase in cases of microcephaly in Brazil was first reported in September 2015, and was followed by retrospective reports of a similar increase seen in French Polynesia after a Zika outbreak there in 2013 and 2014. Now, preliminary reports out of Columbia indicate a similar pattern.

Researchers arrived at the causal relationship by using two approaches to identify potential teratogens: the identification of a combination of rare exposure and a rare defect, known as the astute clinician approach, and the use of epidemiologic data, both of which are central factors of Shepard’s Criteria. The researchers noted that they were unable to identify any alternative hypotheses that could explain the increase in cases of microcephaly in Brazil, French Polynesia, and now Columbia.

The recognition of this link now presents a whole new set of questions that will need to be answered, including the spectrum of defects caused by prenatal Zika infection, the degree of relative and absolute risk of adverse outcomes in fetuses whose mothers were infected at different time points during pregnancy, and other factors that may influence a woman’s risk of adverse pregnancy or birth outcomes.

“Establishing this causal relationship between Zika and fetal brain defects is an important step in driving additional prevention efforts, focusing research activities, and reinforcing the need for direct communication about the risks of Zika,” the CDC said.

The confirmation of the link between Zika and microcephaly does not change the CDC’s current guidelines for prevention, which advise that pregnant women should avoid areas where Zika is active, and that pregnant women who do plan to travel to an affected area take precautions to prevent mosquito bites and sexual transmission of the virus.


  1. Rasmussen SA, Jamieson DJ, Honein MA, Petersen LR. Zika Virus and Birth Defects – Reviewing the Evidence for Causality. N Engl J Med. 2016; doi:10.1056/NEJMsr1604338.
  2. CDC Concludes Zika Causes Microcephaly and Other Birth Defects. CDC Newsroom. Published April 13, 2016. Accessed April 14, 2016.