▶ Watch Video: U.S. prepares for expected imminent arrival of COVID-19 Omicron variant

The Centers for Disease Control and Prevention (CDC) over the weekend began offering passengers coming from Southern African countries free COVID-19 tests as they passed through customs checkpoints at four international airports, in an effort to ramp up surveillance for potential cases of the Omicron variant

Sequencing the variant that caused any positive tests collected from those travelers at the airport will take up to three days, with passengers sent off with additional at-home tests they were asked to send in after three to five days.

The unplanned expansion of the CDC’s airport pilot program is one of a handful of efforts over the past months that the agency has backed to search for new variants of concern like Omicron, which health officials acknowledge could already be spreading within America’s borders. 

It comes as the Biden administration is mulling stepping up new restrictions on international travel as other countries have already done, in an effort to slow the spread of a strain that has already been reported across multiple continents in more than 20 countries and territories.

“As we have done throughout the pandemic, CDC is evaluating how to make international travel as safe as possible, including pre-departure testing closer to the time of flight and considerations around additional post-arrival testing and self-quarantines,” CDC Director Rochelle Walensky told reporters on Tuesday.

First launched in September to target travelers from India at airports in San Francisco, New York and Newark, the CDC-funded program from XpresCheck and Ginkgo Bioworks had so far tested and sequenced samples from thousands of travelers at the three international airports.

“What we were looking to do was test methods, protocols, incentives, and find the best way to get a high level of participation from travelers after a long flight,” Doug Satzman, CEO of XpresSpa, told CBS News. 

The companies were wrapping up the final week of their pilot program when the agency asked them to launch for the Omicron variant, offering tests to international travelers coming off flights Sunday at a fourth airport: Atlanta’s Hartsfield-Jackson International Airport, which ranks as the nation’s busiest.

“So while a lot of my team enjoyed turkey, they were actively lining up our field staff, making sure our partner Ginkgo Bioworks was able to get us enough tests on site. People were literally flying on planes with boxes full of tests,” said Satzman. 

Not all travelers coming off planes can be persuaded to take the airport tests or the at-home follow-up tests, Satzman acknowledged. Unlike the federal pre-departure testing requirements, tests after arriving in the U.S. are currently voluntary. 

And while the program ramped up within days of South African health authorities sounding the alarm over the variant on November 25, Dutch health authorities said they identified Omicron in tests collected there as early as November 19. In Scotland, officials said some cases caused by the variant there had no links to recent international travel.

Spotting America’s first Omicron case instead may come from an array of public health and commercial labs which the CDC has funded to scale up around the country, largely sequencing tests sampled from cases in the community. 

The system can detect circulating viruses down to 0.01% prevalence, according to the CEO of the Association of Public Health Laboratories. For now, the CDC’s latest sequencing report still estimates that virtually all U.S. cases are being caused by the Delta variant.

“We have the tools and surveillance in place to identify the Omicron variant. We also have the tools to prevent Omicron from increasing the strain on our society and our healthcare system,” Walensky said on Tuesday.