Wednesday, April 13, 2016

NEJM: CDC Concludes Zika Causes Microcephaly & Other Birth Defects

Credit CDC


Although the evidence has been accumulating for several months, and a couple of weeks ago the WHO stated there was a `strong scientific consensus' that the Zika virus is a cause of microcephaly, the CDC has been understandably cautious about declaring causality.

After all, we've never seen an arbovirus cause this sort of fetal injury before.

Today, however, at a 2:30pm press conference, the CDC's Director Dr. Tom Frieden (along with a panel of experts) announced that a CDC study - published today in the NEJM - has accumulated enough evidence to dispel any doubts that the Zika virus is a cause of microcephaly and other birth defects.

There is still much that is unknown about the impact of Zika virus infection on developing fetuses, including the full spectrum of neurological damage that maternal Zika infection may cause, the absolute and relative risk of birth defects and at what stages of pregnancy, and determining whether there may be additional co-factors that may contribute to fetal brain injury.

At 5pm CDC Director Tom Frieden announced their findings on twitter:

Here is the CDC's media statement (embargoed until 5pm EDT), followed by a link to the NEJM study.

CDC concludes Zika causes microcephaly and other birth defects

Scientists at the Centers for Disease Control and Prevention (CDC) have concluded, after careful review of existing evidence, that Zika virus is a cause of microcephaly and other severe fetal brain defects. In the report published in the New England Journal of Medicine, the CDC authors describe a rigorous weighing of evidence using established scientific criteria.

“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,” said Tom Frieden, M.D., M.P.H., director of the CDC. 
“We’ve now confirmed what mounting evidence has suggested, affirming our early guidance to pregnant women and their partners to take steps to avoid Zika infection and to health care professionals who are talking to patients every day. We are working to do everything possible to protect the American public.”

The report notes that no single piece of evidence provides conclusive proof that Zika virus infection is a cause of microcephaly and other fetal brain defects. Rather, increasing evidence from a number of recently published studies and a careful evaluation using established scientific criteria supports the authors’ conclusions.
The finding that Zika virus infection can cause microcephaly and other severe fetal brain defects means that a woman who is infected with Zika during pregnancy has an increased risk of having a baby with these health problems. It does not mean, however, that all women who have Zika virus infection during pregnancy will have babies with problems. As has been seen during the current Zika outbreak, some infected women have delivered babies that appear to be healthy.
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. While one important question about causality has been answered, many questions remain. Answering these will be the focus of ongoing research to help improve prevention efforts, which ultimately may help reduce the effects of Zika virus infection during pregnancy.
At this time, CDC is not changing its current guidance as a result of this finding. Pregnant women should continue to avoid travel to areas where Zika is actively spreading. If a pregnant woman travels to or lives in an area with active Zika virus transmission, she should talk with her healthcare provider and strictly follow steps to prevent mosquito bites and to prevent sexual transmission of Zika virus. We also continue to encourage women and their partners in areas with active Zika transmission to engage in pregnancy planning and counseling with their health care providers so that they know the risks and the ways to mitigate them.

As mentioned previously, the CDC held a press conference, with a Q&A session on today's announcement.  Taking part were:

  • CDC Director Tom Frieden, MD, MPH
  • Sonja A. Rasmussen, MD, MS; Director of the Division of Public Health Information and Dissemination, and Editor-in-Chief of CDC’s Morbidity and Mortality Weekly Report (MMWR)
  • Denise J. Jamieson, MD, MPH, CAPT, USPHS; Co-Lead, Pregnancy and Birth Defects Team, CDC Zika Virus Response Team
  • Margaret A. Honein, PhD MPH; Chief, Birth Defects Branch, National Center for Birth Defects and Developmental Disabilities
  • Lyle R. Petersen, MD, MPH; Director, Division of Vector Borne Infectious Diseases, National Center for Emerging and Zoonotic Infections

The transcript & audio should be posted tomorrow or Friday on

The link to the peer-reviewed NEJM article follows:

special report 
Online First

Zika virus has spread rapidly throughout the Americas over the past year. In this report, CDC authors determine that the evidence level has exceeded the threshold to assign causation between prenatal exposure and microcephaly and to declare Zika virus a teratogen.

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