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After careful review of existing evidence, scientists at the Centers for Disease Control and Prevention (CDC) have concluded that Zika virus causes microcephaly and other severe fetal brain defects.

In a report published in the New England Journal of Medicine on April 13, the CDC authors weighed the evidence to establish a causal relationship between Zika virus infection and microcephaly.

“This study marks a turning point in the Zika outbreak.  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,” Tom Frieden, director of the CDC said in a release.

The conclusion was not based on any single piece of evidence but on review of all available evidence in the literature. The authors note that the evidence to draw this causal relationship included three main factors — Zika virus infection during pregnancy and the observed defects in brain development; a specific, rare phenotype involving microcephaly and associated brain anomalies in fetuses or infants with presumed or confirmed congenital Zika virus infection; and strongly biologic plausibility arising from the identification of Zika virus in the brain tissue of affected fetuses and infants.

It is important to note that though CDC has confirmed the causal relationship between Zika virus and microcephaly and other severe brain defects, pregnant women who are infected with the virus are only at a higher risk of having babies with such health problems.  It does not mean that all pregnant women with Zika virus infection will have babies with such problems.

The biggest gain in moving from a hypothesis to definite causal relationship between Zika infection and microcephaly is that direct communication of risks becomes easier, measures needed to prevent infection will get intensified, and efforts to develop diagnostic methods and vaccines against Zika virus infection will get a much needed boost.

Now that the causal relationship has been established, all research efforts can be directed at “understanding the full spectrum of defects caused by congenital Zika virus infection”. Second, it allows for knowing the relative and absolute risk among infants born to mothers who had the infection during pregnancy. Third, efforts can be directed at knowing if other factors like confection with another virus and/or pre existing immune response to another flavivirus are responsible for some babies born to mother infected with Zika virus developing microcephaly.

“Addressing these issues will improve our efforts to minimize the burden of the effects of Zika virus infection during pregnancy,” the authors note.

Published in The Hindu on April 14, 2016