People who have traveled to places where the Zika virus is circulating should wait a month before donating blood, just to be safe, the Food and Drug Administration said Tuesday.
It's the latest just-in-case measure announced by U.S. health officials, who are watching the rapid spread of Zika across Latin America and the Caribbean.
"We are issuing this guidance for immediate implementation in order to better protect the U.S. blood supply," said Dr. Luciana Borio, the FDA's acting chief scientist.
Even if Zika isn't around, people who have Zika-like symptoms, such as a rash, should put off donating blood for four weeks, the FDA said.
"Individuals considered to be at risk include: those who have had symptoms suggestive of Zika virus infection during the past four weeks, those who have had sexual contact with a person who has traveled to, or resided in, an area with active Zika virus transmission during the prior three months, and those who have traveled to areas with active transmission of Zika virus during the past four weeks," the FDA said in its new guidance.
"In areas with active Zika virus transmission, the FDA recommends that whole blood and blood components for transfusion be obtained from areas of the U.S. without active transmission," the guidance reads.
Puerto Rico and the U.S. Virgin Islands, as well as Samoa, have active Zika transmission. They are all U.S. territories. There's no active Zika transmission yet in any U.S. states, although more than 50 travelers have been diagnosed.
"Based on the best available evidence, we believe the new recommendations will help reduce the risk of collecting blood and blood components from donors who may be infected with the Zika virus," said Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research.
The FDA also says it will issue guidance on donating human cells, tissues, and products made using them.
Earlier Tuesday, experts said they still are not sure how long Zika stays in the blood but said it's unlikely to be much longer than 10 days in most cases. But they said the studies have yet to be done that will show that for sure.
They also said they doubted there would be widespread transmission of Zika in the U.S.
"We don't think it will be an outbreak in the United States, but nonetheless, precautions have to be taken," Dr. Marcos Espinal of the Pan American Health Organization told a gathering organized by the National Institute of Medicine in Washigton, D.C.
"The bottom line is we have not seen the peak of this outbreak in Latin America and the Caribbean."
Evidence is building that Zika causes birth defects including microcephaly, characterized by a damaged and underdeveloped brain, other brain damage and a paralyzing condition called Guillain-Barre syndrome.
The World Health Organization recommends aggressive mosquito control measures as the quickest way to control the virus and its spread.