Scientists at the Centers for Disease Control and Prevention (CDC) have concluded, after careful review of existing evidence, that the 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," said Tom Frieden, M.D., M.P.H., director of the CDC. "It is now clear that the virus causes microcephaly."

A statement from the World Health Organization (WHO) backs up these findings and suggests there is scientific consensus that the Zika virus not only causes microcephaly but also Guillain-Barré syndrome:

“While intense efforts continue to reinforce and refine the link between Zika virus and a range of neurological disorders within a rigorous research framework, a stream of recent case report studies as well as a small number of case control and cohort studies, support the conclusion that there exists an association between Zika and microcephaly and Guillain-Barré syndrome..”

“Scarier Than We Thought”

Anne Schuchat, deputy director at the CDC, told CNN that, "Everything we look at with this virus seems to be scarier than we initially thought."

To that end, the CDC is launching further studies to determine whether children who have microcephaly born to mothers infected with the virus is the "tip of the iceberg" regarding damaging effects on the brain and other developmental problems among newborns.

In addition to microcephaly, other problems that have been detected include eye defects, hearing loss, and impaired growth.

Brazil: Ground Zero for the Study

Since May 2015, Brazil has experienced a significant outbreak of Zika virus. In recent months, Brazilian officials reported a 20-fold increase in the number of babies born with microcephaly.

Based on the initial reports of a link between Zika and microcephaly, researchers across the world began studying the connection between Zika during pregnancy and microcephaly, which led to the conclusion drawn by 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," Frieden said. "We are working to do everything possible to protect the American public."

The report said that no single piece of evidence provides conclusive proof that Zika is a cause of microcephaly and other fetal brain defects. Rather, the conclusion was drawn based on increasing evidence from several recently published studies accompanied by careful evaluation using established scientific criteria.

The finding does not mean that every woman who contracts Zika virus during pregnancy will have a baby with problems, only that the likelihood is greater.

"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," said the CDC in a prepared statement.

"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."

CDC Recommendations for Pregnant Women

The CDC recommends that 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 prevent sexual transmission of Zika virus.

Also, the CDC continues 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.

What We Do Know and What We Don’t

Here's what we know for certain, according to the CDC:

  • Pregnant women can be infected with Zika virus;
  • The primary way that pregnant women get Zika virus is through the bite of an infected mosquito; 
  • Zika virus can be spread by a man to his sex partners;
  • A pregnant woman can pass Zika virus to her fetus;
  • Zika virus can be passed from a pregnant woman to her fetus during pregnancy or at delivery.

Here's what we do not know:

  • How likely a pregnant mother is to get Zika;
  • If infected, how the virus will affect her or her pregnancy;
  • How likely it is that Zika will pass to the mother’s fetus;
  • If the fetus is infected, whether or not it will develop birth defects;
  • When in pregnancy the infection might cause harm to the fetus;
  • Whether or not the baby will have birth defects; 
  • If sexual transmission of Zika virus poses a different risk of birth defects than mosquito-borne transmission.

Effect on Future Pregnancies

The CDC says that, based on the available evidence, Zika virus infection in a woman who is not pregnant would not pose a risk for birth defects in future pregnancies after the virus has cleared from her blood.

“From what we know about similar infections, once a person has been infected with Zika virus, he or she is likely to be protected from a future Zika infection,” the CDC said.