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WASHINGTON (AP) — More than 400 people in the District of Columbia who were told last year they didn’t have the Zika virus are being retested because of a human error in the testing process, federal and city officials said Thursday, and two pregnant women were told after subsequent tests that they may have had Zika after all.

The faulty tests were conducted between July and December at the District’s public health laboratory. Dr. Anthony Tran, who heads the lab, told reporters Thursday that he contacted the Centers for Disease Control and Prevention after he became concerned that too many of the results were negative.

The tests were botched because someone at the lab made a math error, the city’s Department of Forensic Sciences said in a statement.

Tran decided to retest all 409 samples, 294 of which were taken from pregnant women. Those tests are being conducted by the CDC, while the other 115 samples from non-pregnant women and men are being tested at other public health labs. Of the samples checked by the CDC, 60 have come back negative for Zika so far, and two have tested positive, officials said.

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With those positive tests, there’s no way to know if the women had Zika or some other virus like dengue that is transmitted by a similar kind of mosquito, said Wendy Kuhnert-Tallman, who co-leads the CDC’s Zika lab task force.

“We can’t say for sure that there was a missed case of Zika,” Kuhnert-Tallman said.

Zika can cause severe birth defects, including babies born with abnormally small heads. The mosquito-borne virus can also be transmitted sexually.

City officials did not say whether the women who were informed of the positive tests have given birth. Pregnant women have been urged to get tested for Zika if they or their sexual partners traveled to areas where mosquitoes carry the virus.

Results from all the follow-up tests are expected in three to four weeks, city officials said.

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This story has been corrected to say not all of those who were retested were women.

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AP Medical Writer Mike Stobbe in New York City contributed to this report.