Just hours before he was set to board a flight home to Argentina on Saturday, Santiago Solans Portillo received some news that appeared to throw a wrench in his travel plans: His coronavirus test had come back positive, authorities say.

But when the 29-year-old arrived at the airport in Miami, he made no such disclosure to the American Airlines agents checking him in, instead presenting a medical certificate that said he was fit to fly.

It was only the following day, when he landed in Buenos Aires and health officials took his temperature, finding he had a fever of 101.3 degrees, that he made his confession: He probably had covid-19 — and should not have boarded the plane.

“Due to this irresponsible, selfish behavior, 200 people are at risk despite having done the right thing while traveling,” Florencia Carignano, Argentina’s top immigration official, told reporters this week.

Proof of vaccination to travel or attend school is not new, but the coronavirus has introduced a potential need to modernize outdated paper standards. (Jonathan Baran/The Washington Post)

As with many other countries, Argentina requires international travelers coming from the United States to present proof of having recovered from covid-19 recently or a negative test to board a flight. Borders have largely been closed to almost anyone besides citizens and permanent residents.

New cases in Argentina have skyrocketed in recent weeks, surpassing a total of 3 million and reaching an all-time high of new daily cases late last month as hospitals scramble to accommodate the influx of covid-19 patients.

But as in much of Latin America, vaccine rollout efforts have stalled — in part because of a shortage of supplies largely stockpiled by the United States and other wealthier countries. About 7 million people, or 16 percent of the total population, had received at least one shot as of May 3, according to data tracked by The Washington Post.

Anxious and tired of waiting, some of the richest Argentines have instead looked north, flying to Miami to get shots. Officials said that Portillo, who owns a commercial pressure-washing business, was one of them.

It is unclear whether Portillo ever did get vaccinated. But Argentine authorities say that he received a medical certificate at some point on Saturday from a clinic in Hollywood, Fla., stating that he was in good health and able to travel internationally.

But in an interview with an Argentine radio station this week, Juan Manuel Dragani, a lawyer in Buenos Aires who represents that clinic, insisted that the certificate was not counterfeit or fraudulent and had been issued to the patient in accordance with all U.S. guidelines.

“There’s a direct responsibility for the accused,” Dragani said, noting that Portillo’s only contact with the clinic had been through a telehealth appointment.

Still, at 5:15 p.m. on Saturday, Portillo received a PCR test that showed he was positive for the coronavirus, authorities said. By midnight, he had boarded American Airlines Flight AA931, and on Sunday morning, the plane — carrying 258 other passengers and 12 crew members — landed in Argentina.

After acknowledging his alleged ruse, authorities said, he was arrested and transported to a Buenos Aires sanitary hotel — one of several facilities that the city had run for travelers arriving from abroad who were ordered to quarantine.

In a brief phone interview Tuesday with Clarín, the country’s largest newspaper, Portillo said he was running a fever and was receiving medical treatment while in custody there.

He claimed that the publication’s previous reporting on his case was “all a lie” and refused to offer more details without consulting his lawyer. (As of that morning, however, court officials noted that he had yet to retain an attorney; The Post was unable to reach Portillo.)

Carignano, the immigration official, said that Portillo could face 3 to 15 years in prison under an Argentine law that bars people from knowingly exposing others to infectious diseases. She noted that his “complex judicial situation” would be compounded if any of his fellow passengers develop symptoms in the coming days.

Buenos Aires city health officials have tracked down at least 14 people who were sitting near Portillo on the plane, ordering them to quarantine for at least seven days, although it is unclear whether any have tested positive for the virus.

A judge has seized Portillo’s cellphone to examine when he received the medical certificate, and at what time a different clinic in Miami notified him that he had tested positive for the virus.

Carignano has also said that her agency would sanction and fine American Airlines for allowing a sick person to board the flight, in violation of the country’s coronavirus rules. The airline did not immediately respond to a request for comment from The Post.

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To some, carrying a card to verify you've had a vaccination seems like a foreign concept. But vaccine cards, or yellow cards, have been used for decades. (Allie Caren/The Washington Post)