Zika virus has been detected in units of donated blood in Florida, but testing of individual donations, which will start soon in Washington state, kept the virus out of the blood supply.

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Zika virus has been detected in units of donated blood in Florida, federal health officials said Tuesday.

But the number so far is small, and new testing in high-risk areas kept the virus from entering the U.S. blood supply.

Food and Drug Administration officials confirmed that “a few” units of donated blood tested positive in Florida, the first state with local mosquito transmission of Zika virus, which causes devastating birth defects and other problems.

“A few additional potential positive donations are also currently under investigation,” Tara Goodin, an FDA spokeswoman, said in an email.

The virus was detected by screening each collected unit using FDA-approved investigational tests.

The FDA would not say exactly how many units tested positive, or when and where those donations were collected.

Goodin referred questions to firms that make or supply the tests: Hologic, Grifols and Roche. Company officials either did not respond or could not provide details.

It’s the first report of multiple positive tests in a U.S. state. The FDA in August said one donated unit of blood in Florida had tested positive for Zika virus and was removed from the blood supply,

Such results were expected eventually, said Dr. James AuBuchon, president and chief executive of Bloodworks Northwest, a Seattle blood-collection agency.

“It wouldn’t be surprising,” he said. “That’s why the testing began.”

Zika virus is primarily spread by mosquitoes but can be spread through sex and blood transfusions.

In Puerto Rico, about 1 percent of blood donations tested positive for the virus, federal officials said.

In Florida, blood donations were halted in July in Miami-Dade and Broward counties until Zika testing could begin.

FDA officials in August called for all U.S. blood centers to begin screening for Zika virus. States at high risk for local mosquito transmission of the virus, including Florida, were first urged to test individual units of donated blood, followed by states like Washington, which are at lower risk. The FDA set a Nov. 18 deadline.

AuBuchon on Monday criticized the American Red Cross for a plan to delay testing of individual units in low-risk areas. Instead, they’ll rely on testing pooled units of blood in those regions until the end of the year. Such pooled testing may fail to detect up to 25 percent of Zika-infected blood, AuBuchon said.

“This shows that the individual donation testing protocol does work, it does pick up potentially infectious units,” AuBuchon said. “If you’re going to do the testing, you should do it right.”

Red Cross officials strongly objected to AuBuchon’s criticism. They said they’re working closely with the FDA and that federal officials have no objection to the timeline.

Zika virus has been confirmed in nearly 4,000 people, mostly travelers, in the continental U.S.

Nearly 26,000 have been infected in U.S. territories, according to the Centers for Disease Control and Prevention.