(CNN) — The Biden administration’s new, stricter Covid-19 testing requirements for all travelers coming to the United States will take effect on Monday, an administration official told CNN.

The new rules will require each traveler flying into the US from another country to test negative one day before their departure, changing rules that had allowed inbound travelers to test up to three days before entering the country. The new rule from the US Centers for Disease Control and Prevention will take effect at 12:01 a.m. ET on Monday.

The shift in policy — which President Joe Biden announced Thursday alongside a slate of new steps to combat Covid-19 this winter — underscores the potential threat posed by the newly discovered Omicron variant. Scientists are still working to determine how transmissible the variant is, how sick it makes people and how well the current vaccines work against it.

“Experts say that Covid-19 cases will continue to rise in the weeks ahead and this winter,” the President said. “So we need to be ready.”

Any foreign national who travels to the US must be fully vaccinated, though there remains no vaccination requirement for American citizens traveling via air, either globally or domestically. The White House said earlier in the day that a vaccine requirement for domestic travel remained on the table as an option for the future.

On Monday, the US banned all travel from South Africa and seven neighboring nations, with the exception of US citizens and legal permanent residents, who must test negative to enter the US but not once they have arrived. Asked Tuesday how long current travel restrictions would remain in place, Biden said, “Well, it kind of depends.”

“It’s going week-to-week, to determine what we need and what the state of affairs (is). We’re going to learn a lot more in the next couple of weeks about the lethality of this virus, about how much it spreads, about whether what we have can control it, etc.,” he continued.

Health officials are urging people to get vaccinated against Covid-19 or get a booster if they’re eligible. Other measures such as masks, handwashing, physical distancing and ventilation will still work against the Omicron variant.

The Delta variant of the coronavirus remains the dominant variant globally and in the United States.

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