Government health officials warned Thursday that many pregnant women infected with Zika may have fallen under the radar, and they're seeing an uptick in birth defects in areas that saw local Zika activity.
The incidence rate of birth defects "strongly linked" to Zika infection in areas with local transmission of the virus—which include South Florida, parts of south Texas and Puerto Rico—rose by 21% in the latter half of 2016 compared to the first half of that year.
Birth defects considered "strongly linked" to Zika infection include such conditions as brain abnormalities, microcephaly, eye abnormalities and central nervous system dysfunction.
The findings were published Thursday in the Centers for Disease Control and Prevention's Morbidity and Mortality Weekly Report.
The analysis marked the agency's first large-scale population-based surveillance of birth defects possibly associated with Zika. Researchers monitored activity across 15 states and territories to collect data on a population of approximately 1 million live births.
Most of the mothers who had babies with Zika-related birth defects were found to not have laboratory evidence of virus infection either because they were never tested, not tested at the right time or were not believed to have been exposed to Zika.
The report found that approximately three out of every 1,000 babies born in the 15 states and territories studied in 2016 had a birth defect possibly associated with Zika virus infection in the mother. Roughly half of all birth defects identified involved children born with either brain abnormalities or microcephaly (smaller than normal head size).
The number of Zika cases in the 50 states has dropped dramatically over the past two years, falling from its peak of more than 5,100 cases reported in 2016 to more than 400 in 2017. There were no reported cases of Zika in 2018 as of Thursday, according to the CDC. However, Los Angeles County health department officials reported earlier this month that they have identified their first confirmed Zika case of the year in a woman who contracted the virus through sexual transmission.
Between January 2016 and December 2017, there were more than 7,000 pregnancies with evidence of possible Zika virus infection in U.S. states and territories.
Report authors stressed the need for continued surveillance for possible birth defects related to Zika virus infections since most of the pregnant women exposed to the virus in 2016 gave birth in 2017, which researchers expect will lead to another rise in Zika-related birth defects once surveillance for the past year is completed.
The report recommended communities in the most heavily affected areas use the data to plan for the distribution of needed resources to help patients and families.
"Babies with Zika-related birth defects need all the help they can get, as soon as possible and for as long as they need it," CDC Director Dr. Brenda Fitzgerald said in a written statement. "This report highlights the critical importance of documenting birth defects possibly related to Zika and our need to maintain vigilance."