A federal agency is closing a legal loophole that allowed U.S.-based blood plasma companies to harvest plasma from thousands of Mexicans a day, who were lured by bonus payments and hefty cash rewards, as a 2019 ProPublica and ARD German TV investigation showed.

U.S. Customs and Border Protection announced on June 15 that effective immediately, it would no longer permit Mexican citizens to cross into the U.S. on temporary visas to sell their blood plasma. A statement provided to ProPublica and ARD said that donating plasma is now considered “labor for hire,” which is illegal under the visitor visa most border residents use to cross into the United States to make donations.

The U.S.-Mexico border is still mostly closed to “nonessential travel” due to the COVID-19 pandemic, and the Biden administration has said those restrictions will remain in place through at least July 21. The travel restrictions have greatly reduced the cross-border plasma business. However, Paul del Rincon, a customs chief based in Eagle Pass, Texas, estimated in an interview posted to Facebook with broadcaster La Rancherita del Aire that even during the pandemic, 300 to 400 people crossed daily to donate plasma. In other border cities, like El Paso, Texas, donors have not been allowed to cross since the restrictions went into effect.

Before the pandemic, donors could make up to $40 a donation and over $4,000 a year for those who donated as often as possible. U.S. law caps donations at 104 a year, compared to Europe’s recommended frequency of 33 times per year. In Mexico, selling plasma is entirely illegal.

However, with COVID-19 causing a 20% decrease in plasma donations in 2020, according to the industry group the Plasma Protein Therapeutic Association, prices have soared. Plasma centers in El Paso offered as much as $700 a month for twice-weekly donations in summer 2020, according to the El Paso Times. At the beginning of June, Facebook posts by plasma centers showed offers of up to $1,000 a month.

“We know a lot of people depend on what they receive from selling plasma to support themselves in Mexico,” del Rincon said. “And we know the plasma centers also count on them. And this is going to hurt them.”

The U.S. is the biggest global exporter of blood plasma — a market that reached $21 billion in 2019 — and plasma centers openly relied on cross-border donations to keep their supplies up. The industry group told ProPublica and ARD that they plan to lobby against the new restriction: “The Plasma Protein Therapeutics Association looks forward to working with CBP and the Biden administration to quickly reverse this policy,” a spokesperson wrote.

However, as ProPublica and ARD found, frequent plasma donation was also hurting the Mexican citizens who relied on the system for money. Frequent donors were underweight and showed low levels of antibodies.

The B1/B2 visitor visa used most often by Mexican border residents permits some business activity, but it does not permit Mexican citizens to work in the U.S. Before the new announcement, plasma donation fell into a legal gray area, with some CBP agents refusing to let people cross for donations but others allowing it.

Del Rincon told La Rancherita del Aire that in most cases people with appointments to donate plasma would just be asked to return to Mexico. However, he said, they risked losing their visas if they heard about the new instructions and went ahead with plasma donations anyway. “What’s important is that people not put their visas at risk,” del Rincon said.

It’s not clear how CBP will enforce the new policy beyond simply asking people why they’re crossing. Even before the guidance was issued, regular plasma donors often lied to agents about the purpose of their visits, claiming they were going shopping or visiting relatives.

Mexican residents are required to present their B1/B2 visa Border Crossing Cards at the plasma center when they donate, so plasma centers will know when donors are violating the new policy. Grifols, a company that operates several border plasma centers, answered what it said were “hundreds” of messages from donors on its Spanish-language Facebook page last week: “The answer is that for the moment (indefinitely) you can’t donate plasma,” the company wrote. But if any plasma centers do continue to accept cross-border donations in violation of the new policy, it’s not clear whether the U.S. would crack down.

U.S. Citizenship and Immigration Services told ProPublica and ARD in 2019 that companies could face criminal charges if they engaged in a “pattern or practice of knowingly hiring” people who aren’t authorized to work in the U.S. — including B1/B2 visa holders. However, the CBP statement doesn’t mention any consequences for plasma centers if they violate the new policy.