Officials at the Centers for Disease Control and Prevention have determined that Zika virus infection is a cause of microcephaly in babies born to infected mothers, following a systematic review of the latest research on Zika virus.

The CDC released findings from that review in a peer-reviewed special report published online April 13 in the New England Journal of Medicine (2016. doi: 10.1056/NEJMsr1604338 ). The report, CDC officials said, incorporated evidence from as recently as the past week.

In a press conference in April, CDC director Tom Frieden said there was “no longer any doubt” that Zika virus causes microcephaly. Dr. Frieden’s statements reflect what appears to be a growing scientific consensus. On March 31, the World Health Organization reported that there was a “strong” consensus that Zika virus can cause microcephaly, Guillain-Barré Syndrome, and other neurological disorders. New microcephaly cases in Colombia – with 32 reported by the end of March – are among the findings cited by both the WHO and the CDC.

Dr. Frieden stressed that no single piece of evidence was seen to provide conclusive proof of causation, but that the CDC scientists’ conclusions were based on “a thorough review of the best available evidence” subjected to established criteria.

For its review published in the New England Journal of Medicine, CDC scientists led by Dr. Sonja Rasmussen subjected available evidence to two separate criteria to determine the relationship of Zika virus to the spikes in microcephaly cases seen in countries where Zika is spreading. Shepard’s criteria were used to prove teratogenicity, and the Bradford Hill criteria were used to show evidence of causation.

The CDC has not made any changes to its guidance on Zika virus prevention, which includes advising pregnant women to avoid travel to regions where Zika transmission is occurring, take steps to prevent infection if they live in areas where Zika virus is present, and use condoms to prevent sexual transmission of Zika from a partner. On April 13, the CDC added St. Lucia to its growing list of countries to be avoided by pregnant women.

The full spectrum of defects caused by congenital Zika infection is still poorly understood. Additional studies are underway at CDC, Dr. Frieden said, to better understand the severe phenotype of microcephaly seen in babies born to Zika-infected mothers and “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.”