"It is now clear that Zika does cause microcephaly," Dr. Tom Frieden, director of the U.S. Centers for Disease Control and Prevention, said in opening a news conference Wednesday at CDC headquarters in Atlanta.
Frieden went on to say that the CDC is launching further studies to determine whether children who have microcephaly caused by the Zika virus might have other brain and developmental problems.
Microcephaly might just be "the tip of the iceberg" when it comes to Zika, even in infants who appear to be normal at birth, according to Dr. Sonja A. Rasmussen, the principal author of the new study.
Microcephaly, a condition in which a baby is born with an unusually small head and small brain, has been linked to Zika since February when World Health Organization director Margaret Chan declared the virus "guilty until proven innocent" in relation to the rare birth defect.
This is the first time the virus has been scientifically proven to be the cause, as well as the first time a mosquito-borne virus has been shown to cause birth defects.
FILE - Geovane Silva holds his son Gustavo Henrique, who has microcephaly, at the Oswaldo Cruz Hospital in Recife, Brazil, Jan. 26, 2016.
Scientists involved in the study say it will drive additional prevention efforts, focus research on a vaccine, and reinforce the need to educate people about the risks of Zika.
The CDC study was published in the New England Journal of Medicine. The study pointed out that the Zika virus has spread rapidly in the Americas since first being identified in Brazil in early 2015, where it was linked to adverse pregnancy and birth outcomes.
Rasmussen said the study looked at several criteria and reviewed existing evidence, including whether there were specific defects or patterns of defects that point to a causal relationship to Zika, whether pre-natal exposure to the virus can result in birth defects, and whether the link makes sense biologically.
Rasmussen said the criteria showed all links to be true, and showed that the babies included in the study had severe microcephaly with unusually small heads.
However, there are still a lot of unknowns.
"We don't know the full range of health problems the Zika virus will cause,” Rasmussen said. “Will it cause learning problems later in life? Are other factors involved, such as another infection?"
Other unknowns: why the virus causes microcephaly in some infants but not in others; whether infants born to pregnant women who had the virus but did not have the symptoms will have the same risk of microcephaly as those whose mothers had symptoms of the virus; and how long these children can expect to live.
While a lot is known about babies with microcephaly, "we don't know a lot about babies with Zika-caused microcephaly," Rasmussen said.