The FDA is advising all U.S. states and territories to screen donated blood and blood components for the Zika virus, a major expansion of screening efforts to prevent spread of the mosquito-borne virus through blood supplies.

Previously, the agency had also advised screening at blood banks in Puerto Rico and parts of Florida, where there have been cases of local Zika transmission. That was before a report from Brazilian researchers and the American Red Cross published in the New England Journal of Medicine said a donor had transmitted Zika to two hospitalized patients through a blood platelet transfusion in January 2016.

“There is still much uncertainty regarding the nature and extent of Zika virus transmission,” said Peter Marks, MD, PhD, director of the FDA’s Center for Biologics Evaluation and Research. “At this time, the recommendation for testing the entire blood supply will help ensure that safe blood is available for all individuals who might need transfusion.”

The FDA says all parts of the U.S. where active transmission have been confirmed are in compliance with the previous guidance of screening donated whole blood and blood components for Zika, using pathogen-reduction devices, or halting blood collection and instead obtaining donations from areas where the virus isn’t actively being spread by mosquitoes.

This new alert recommends the entire county and U.S. territories use a FDA-authorized blood screening test or a licensed test on donated blood or blood components. A pathogen-reduction device could be used for plasma and other platelet products.

Blood centers could be testing for the virus for quite a while, as the agency said the recommendations “will be in effect until the risk of transfusion transmission of Zika virus is reduced.”

According to the most recent federal statistics, more than 10,000 cases of Zika have been confirmed in the U.S and its territories. The virus, which can be transmitted by mosquitoes or through sexual contact, can cause serious birth defects such as microcephaly, but four out of five adults who are infected never develop symptoms.