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Travelers returning from places where the Zika virus is spreading or people who have shown symptoms of the virus should not donate blood for four weeks, the US Food and Drug Administration said Tuesday.

The FDA also recommended that people who have had sex with partners who have been in places with Zika transmission in the past three months should delay giving blood for 28 days, enough time for the virus to clear the body.

Blood banks in the United States had already been turning away potential donors who had visited Latin American countries where Zika has been transmitted, asking that they wait four weeks after returning. Canada has also announced that donors should defer giving blood if they have traveled to Zika-affected countries.


The FDA said it is taking the precaution in part because 80 percent of people who contract the virus do not show any symptoms, meaning they could be carrying the virus unknowingly.

“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,” FDA official Dr. Peter Marks said in a statement.


Zika, which has spread through most of Latin America after first appearing in Brazil last year, typically causes just a few days of fever, aches, and rash in the people who contract the virus and do show symptoms. But scientists are investigating whether the virus could increase the risk of pregnant women giving birth to babies with abnormally small heads, a condition called microcephaly.

The virus is primarily spread by Aedes aegypti mosquitoes. In the United States, the vast majority of cases have been in people who contracted the virus while traveling abroad and started showing symptoms upon their return.

Are you at risk for contracting Zika virus? Your level of risk depends in part on your living conditions. Alex Hogan/STAT

There are at least two known cases of sexual transmission, however, and Zika can persist in semen long after symptoms of infection have cleared. The FDA said Tuesday it plans to come out with other recommendations for people who donate human cells such as sperm and eggs because of the risk of sexual transmission, which most scientists say is a rare occurrence.