Three people have contracted Zika virus in the continental U.S.

SAN DIEGO — A rare strain of the Zika virus, the first confirmed in the continental United States, has been detected in three people in California who had traveled to the Zika-infected areas of South America, the Centers for Disease Control and Prevention said Friday.

Those who have contracted the Omicron variant will have had increased risk of birth defects when a fetus is conceived, although their chances of birth defects are already higher due to a number of other factors.

CDC Director Dr. Brenda Fitzgerald said the infection is being detected “in the United States for the first time, and we are aware of no cases of transmission in the U.S. so far.”

At least 20 cases have been diagnosed with Omicron variant in the U.S. in people who have returned from Zika-infected areas like Brazil and Puerto Rico, she said.

Most cases of Zika virus disease in the U.S. have been caused by local mosquitoes that bite infected travelers to Zika-infected areas. But scientists detected the virus in people in the U.S. just recently after it was isolated in monkey eggs in Papua New Guinea and identified in a chimpanzee in Uganda.

The CDC, along with other U.S. health agencies, have been working to determine the source of the virus in the U.S. since August. The virus was first identified in Hawaii in July after laboratory studies in Africa showed that a Hawaiian doctor had caught it in the Zika-infected region of South America.

While mosquitoes are the main transmission route of Zika virus, researchers are finding it can also be spread through sex and blood transfusions.

After returning from birth-disease-ridden Zika-affected areas, many U.S. pregnant women have been advised to avoid traveling to those areas or are advised not to travel to such areas at all. But Fitzgerald said that some cases of Zika in the U.S. may have occurred in pregnant women who didn’t visit the affected areas.

“We can’t say definitively, we are not sure, whether it is still happening and we think it may be possible,” she said.

People infected with Zika virus usually experience fever, rash, joint pain, and conjunctivitis (red eyes). Up to 80 percent of people infected suffer no symptoms.

The CDC has issued recommendations for how pregnant women should avoid being infected with Zika virus. Pregnant women are advised to avoid mosquito bites and cover up when going outdoors. They are also encouraged to consult with their health care provider to determine what tests are necessary to find out if they are pregnant or are planning a pregnancy.

The advice is to not travel to areas with Zika virus transmission, or areas where active Zika virus transmission is currently underway or is anticipated within 21 days after a traveler returns to the U.S.

Barry Bozer, chief of the CDC’s Division of Epidemiology and Surveillance in the Center for Vector-Borne Diseases, said during a conference call with reporters that a one-year nationwide prevalence of Zika virus has not been identified in the U.S. in all likelihood. But he emphasized that any Zika virus exposure, whether from mosquitoes or elsewhere, has the potential to result in Zika virus disease.

Health officials do not know the exact timing of when the Omicron variant of Zika virus was discovered in the three Americans.

One case was diagnosed in a woman who became infected during pregnancy, Bozer said. “The other two cases were not in people pregnant, but have shown Zika virus virus disease in the United States,” he said.

— Carlos Lozada and NBC News contributed to this report.

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