The Biden administration is preparing to implement new travel guidelines that would require proof of a negative COVID-19 test within one day of flying into the U.S., including for vaccinated people, a spokesperson from the Centers for Disease Control and Prevention confirmed.

The expected change comes as the country beefs up surveillance for the omicron variant, the first case of which in the U.S. has been identified in California, the California and San Francisco Departments of Public Health said Wednesday. The CDC said the person traveled from South Africa on Nov. 22.

Omicron has been deemed a “variant of concern” by the World Health Organization and had been detected in over 20 countries as of Tuesday.

“CDC is working to modify the current global testing order for travel as we learn more about the omicron variant; a revised order would shorten the timeline for required testing for all international air travelers to one day before departure to the United States,” CDC spokesperson Kristen Nordlund said Tuesday night. “This strengthens already robust protocols in place for international travel, including requirements for foreign travelers to be fully vaccinated.”

Under the current guidelines, people from other countries who are not fully vaccinated cannot travel to the U.S., while people who are fully vaccinated can as long as they provide proof of a negative COVID-19 test within three days of traveling. For unvaccinated Americans, the guidelines already required proof of a negative test within one day of traveling. The potential new rule would expand that one-day requirement to all vaccinated travelers coming into the U.S. from other countries.

For post-travel recommendations, the CDC also suggests vaccinated travelers get tested three to five days after arriving in the U.S. and that unvaccinated travelers stay home to self-quarantine for a full seven days, even if they test negative during that timeframe.

Earlier on Tuesday, the White House confirmed it was considering updates around testing requirements and said policy discussions were ongoing across the government as more is learned about the omicron variant.

More updates on the country’s response to the variant are expected Thursday.

CDC Director Rochelle Walensky outlined some of the expected changes at the White House COVID-19 briefing on Tuesday morning. She said the CDC is analyzing 80,000 COVID-positive tests per week — or about one in seven tests — looking for the omicron variant. The delta variant continues to account for 99.9% of all tests analyzed, Walensky said.

Asked if she was confident in the CDC’s surveillance system given how many other countries had detected the variant before the U.S., Walensky said the system is “robust.”

The director also said the CDC is working on expanding a surveillance program in the nation’s four busiest international airports, John F. Kennedy International Airport, San Francisco International Airport, Newark Liberty International Airport and Hartsfield-Jackson Atlanta International Airport, which would allow for more COVID-19 tests on international arrivals — though there was no indication that testing would be mandatory for arriving travelers or which arriving planes would be offered the tests.

In the meantime, experts are calling on all Americans to get vaccinated if they haven’t yet and to get boosted if they’re over 18 and were fully vaccinated over six months ago. Of those eligible for a booster, 100 million Americans haven’t gotten one yet, the White House said on Tuesday, while just about 20% of fully vaccinated Americans have, the CDC’s vaccine data shows.

Though the data on how transmissible and severe the omicron variant is will not be available for a few more weeks, as scientists around the globe work to gather it, experts believe it’s unlikely it will completely chip away at the protection from vaccines and boosters, particularly when it comes to hospitalization and death.

“Remember, as with other variants, although partial immune escape may occur, vaccines and particularly boosters give a level of antibody that even with variants like delta, give you a degree of protection, particularly against severe disease,” Dr. Anthony Fauci, chief medical adviser to the White House, said on Tuesday.

On Tuesday, Pfizer BioNTech asked the Food and Drug Administration to consider expanding the booster recommendation to include 16- and 17-year-olds. The agency is expected to review the request in the coming weeks.