Zika virus infections appear to be increasing rapidly in Puerto Rico, U.S. health officials said Friday, putting thousands of pregnant women at risk for potential infection.
"Summer is heating up and so is Zika," Tom Frieden, director of the Centers for Disease Control and Prevention, told reporters on a conference call. "This could lead to dozens or hundreds of infants being born with microcephaly in the coming year."
The CDC is tracking Zika infections in the blood supply in Puerto Rico, what Frieden called potentially "our most accurate real-time leading indicator of Zika activity" there. The test is made by Swiss drug giant Roche and has been used to test donated blood in Puerto Rico since early April.
The most recent week of data, through June 11, showed 1.1 percent of blood donations in Puerto Rico tested positive for Zika. This implies a higher rate of infection in the community, he said: "perhaps a 2 percent rate of infection each month."
"This means over the course of many months, i.e., a nine-month pregnancy, there is a substantial chance a woman would become infected," Frieden said.
The numbers are rising, and Frieden said similar viruses transmitted by the same mosquito — dengue and chikungunya — typically peak in Puerto Rico over the summer months and into the fall.
Zika has been proven to cause microcephaly in babies of some pregnant women who become infected. It's a birth defect that leads to underdeveloped brains and smaller heads. Data from other countries up to this point indicate a pregnant woman infected with Zika in her first trimester may have between a 1 and 15 percent chance of having a baby with microcephaly, Frieden said.