SOMERTON, Ariz. (AP) — One by one, a voice called out the names of 169 people just released by U.S. Border Patrol. Migrants rose from folding chairs in a clinic warehouse and walked to a table of blue-robed workers, who swabbed their mouths.

All but two Cuban women tested negative for COVID-19 that February morning. They were quarantined to motel rooms, while other migrants boarded chartered buses to Phoenix’s Sky Harbor International Airport for flights across the U.S.

Theirs were among just seven of 5,301 tests the Regional Center for Border Health near Yuma, Arizona, did last month for released migrants that were positive — a rate of 0.1%

COVID-19 rates are plunging among migrants crossing the border from Mexico as the Biden administration faces a Wednesday deadline to end or extend sweeping restrictions on asylum that are aimed at limiting the virus’ spread. Lower rates raise more questions about scientific grounds for a public health order that has caused migrants to be expelled from the United States more than 1.7 million times since March 2020 without a chance to request asylum.

While there is no aggregate rate for migrants, test results from several major corridors for illegal border crossings suggest it is well below levels that have triggered concerns among U.S. officials.

In California, 54 of 2,877 migrants tested positive the first two weeks of March, according to the state Department of Social Services. That’s a rate of just 1.9%, down from a peak of 28.2% on Jan. 8.

In Pima County, Arizona, which includes Tucson, the seven-day positivity rate among migrants didn’t exceed 1.3% in early March and dropped to 0.9% on March 10. The seven-day rate topped 5% on only two days during the final three months of last year. Then, as the omicron variant spread, it surged to double-digits for most of January, peaking at 19.2% on Jan. 12 and falling below 5% on Feb. 12.

McAllen, Texas, the largest city in the busiest corridor for illegal crossings, has a higher rate among migrants — 11.3% for the week ending March 16 — but it has been consistently lower than the general population. Only two of 24 border counties have had high rates in the general population: Hidalgo, which includes McAllen, and Yuma in Arizona.

The rate among migrants in McAllen peaked at 20.8% the last week of January, when it was double that in the general population. It bottomed at 1.4% the last week of November, when the general population was at 6.2%.

As mask mandates have lifted, the Centers for Disease Control and Prevention is under mounting pressure to fully restore asylum by ending Title 42, named for a 1944 public health law. Critics say it has been an excuse to wriggle out of asylum obligations under U.S. law and international treaty.

Justin Walker, a federal appeals court judge in Washington, wrote this month that it was “far from clear that the CDC order serves any purpose” for public health. Walker, who was appointed by President Donald Trump, noted that the Biden administration hasn’t provided detailed evidence to support the restrictions.

“The CDC’s order looks in certain respects like a relic from an era with no vaccines, scarce testing, few therapeutics, and little certainty,” Walker wrote for a three-judge panel.

CDC Director Dr. Rochelle Walensky noted falling rates when she ended asylum limits on unaccompanied child migrants on March 11, while keeping them for adults and families with kids. In August, U.S. border authorities began testing children traveling alone in their busiest areas: positives fell to 6% in the first week of March from a high of nearly 20% in early February.

The White House and Homeland Security Department have said decisions on Title 42 rest with the CDC. Walensky told reporters Wednesday that the CDC was reviewing data ahead of next week’s deadline, noting that its two-month renewal in late January came near the peak of the omicron variant.

Scientific arguments for Title 42 have met with skepticism from the start.

The Associated Press reported in 2020 that Vice President Mike Pence directed the CDC to use its emergency powers, overruling agency scientists who said there was no evidence it would slow the coronavirus.

Anne Schuchat, the second-highest ranking CDC until last May, told members of Congress after her departure that the asylum limits lacked foundation as a public health measure when introduced.

“The bulk of the evidence at that time did not support this policy proposal,” she said.

Title 42 also has supporters. In a ruling this month in a lawsuit over the order, U.S. District Judge Mark Pittman in Fort Worth, Texas, said: “There should be no disagreement that the current immigration policies should be focused on stopping the spread of COVID-19.”

Even while large-scale expulsions were carried out under Title 42, the U.S. processed more than 2.8 million cases under normal immigration laws, which allow people to seek asylum.

With costs and strained diplomatic relations limiting expulsions to many countries, migrants are often released to nongovernmental groups and ordered to appear later in immigration court. The groups test for COVID-19.

In El Paso, Annunciation House saw positives plunge to around 2% among the roughly 175 migrants it tested daily in early March, said director Ruben Garcia. Positives were close to 40% at the height of the omicron variant, he said.

In Arizona, at the Regional Center for Binational Health, monthly rates peaked at 3% last year.

Still, Amanda Aguirre, its president, is wary about lifting Title 42.

“My concern is that at any time we’re going to see new variants coming into this area,” she said.

The Val Verde Border Humanitarian Coalition, which tests migrants in the busy Del Rio, Texas, area, said it went several weeks without a single positive.

“Yesterday there was one positive and today there was one positive — that’s out of hundreds tested,” the group wrote last week in response to questions.

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Ghisolfi reported from Palo Alto, California. Angeliki Kastanis in Los Angeles contributed.

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This story has been update to correct the date of the renewal decision to Wednesday, not Tuesday.

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