The report Wednesday in the New England Journal of Medicine said nearly 66,000 people, including nearly 12,000 pregnant women, were reported to have Zika virus infections in Colombia from August 2015 though early April, 2016.
Cases of microcephaly are starting to emerge in Colombia. From Jan. 1, 2016 through April 28, 2016, four infants were born with microcephaly and had laboratory-confirmed Zika virus infections. But none of these infants' mothers had symptoms of the disease during pregnancy and were not reported as part of the government's monitoring.
Only about 1 in 5 people with Zika infections show symptoms. The most common symptoms are fever, rash, joint pain or red eyes.
“This is really adding weight to existing data that asymptomatic infection is also associated with microcephaly," said Margaret Honein, chief of the birth defects branch at the Centers for Disease Control and Prevention, and one of the authors of the report.
She noted that the 12,000 pregnant women reported to have Zika are only the ones with symptoms. That means "a lot more" pregnant women infected with Zika, who don't have symptoms, could be at risk, she said.
But the report did offer a glimmer of reassuring news.
Researchers found that among a small group of Zika-infected women, a majority of those infected in the third trimester delivered healthy babies with no apparent abnormalities. A majority of the women who contracted Zika in the first or second trimester were still pregnant at the time of the report in early April.
Other research has found that pregnant women infected with the Zika virus during their first trimester face as high as a 13 percent chance that their fetus will develop microcephaly.
The Colombia data is preliminary, Honein said. Still, "this report has somewhat reassuring news about infection in the third trimester," she said. "While we're not seeing birth defects, we do need to do continued monitoring and follow-up on the infant outcomes."
Researchers are planning to monitor pregnant women in several cities in Colombia to understand how the babies develop and evaluate them for various potential developmental problems besides microcephaly and brain abnormalities, she said.