• The Biden administration is tightening travel rules to and within the U.S., requiring all in-bound international passengers to test for Covid within 24 hours of departure.
• It's also extending its mask requirement on all domestic flights and public transportation through March 18.
• The changes are part of a broader plan to bolster its arsenal of tools in the nation's fight against the virus as the world enters its third year of the pandemic. 

The Biden administration is tightening travel rules to and within the U.S., requiring all in-bound international passengers to test for Covid within 24 hours of departure starting on Monday and extending its mask requirement on planes and public transportation through March 18.

The changes were announced Thursday as part of a broader plan to bolster the nation's arsenal of tools in its fight against the virus as the world enters its third year of the pandemic. 

The plan is also the latest move by the Biden administration to stem the spread of the new, highly mutated omicron variant of the coronavirus, which was first reported to the WHO by South Africa a week ago. At least 23 countries have identified omicron cases so far, and that number is expected to rise in the coming days and weeks, the WHO said Wednesday. 

The U.S. joined that list after confirming its first case of the variant in Northern California on Wednesday. A second case was confirmed Thursday in a Minnesota resident who recently returned from a convention in New York City.

"This variant is a cause for concern, but not panic," President Joe Biden said Thursday afternoon at the National Institutes of Health.

"We're going to fight this variant with science and speed, not chaos and confusion," he continued.

The tightened pre-departure testing protocols will apply to all in-bound international travelers regardless of vaccination status and will begin as early as next week, senior administration officials told reporters during a press call late Wednesday. Previously, the U.S. required proof of a negative Covid test taken within 72 hours of departure, though this was tightened to only vaccinated travelers last month. Unvaccinated travelers had to have a negative Covid test within one day of departure.

"Our doctors believe tightening testing requirements for pre-departure will help catch more cases, potential cases of people who may be positive and inside the country," a senior administration official said. "And so now is the right time to do it. And we can implement it very quickly."

The mask mandate, which was originally supposed to expire in January, requires masking while on buses, trains, planes and transportation hubs such as indoor bus terminals and airports. Fines will remain doubled from their initial levels for noncompliance with the requirement, starting at $500, and going up to $3,000 for repeat offenders.

Biden said such changes "strengthen international travel" and give the U.S. "more time to stop the spread and study new variants."

Vaccine mandates

The Biden administration's new plan also urges businesses to "move forward expeditiously" with requiring workers to get vaccinated or tested weekly. Such protocols are "especially important" given the new omicron variant, the plan added. 

"We're asking businesses to step forward and do what's right to protect our workers, our communities — to put in place some sort of vaccination requirement or testing requirements for the workplace," the senior administration official said during the call. 

The administration had given businesses with 100 or more employees until Jan. 4 to ensure their staff are either vaccinated against Covid-19, or submit a negative test weekly before entering the workplace. Unvaccinated employees were also supposed to start wearing masks indoors at the workplace on Dec. 5.

However, the Occupational Safety and Health administration suspended enforcement and implementation of the requirements earlier this month, after the U.S. Court of Appeal for the 5th Circuit halted the policy pending review. 

More than two dozen lawsuits have been filed to overturn the Biden policy. But labor unions are asking the courts to expand the requirements to cover smaller businesses and protect more workers.

"While my existing federal vaccination requirements have been reviewed by the courts, this plan does not expand or add to those mandates," Biden said during his remarks. "A plan that all Americans hopefully can rally around and it should be, it should get bipartisan support, in my humble opinion.

"It should unite us not continue to separate us," he continued.

Home Covid tests

The Biden administration's winter Covid plan also includes more free at-home tests.

More than 150 million Americans with private insurance will be able to get reimbursed for their at-home tests, according to the plan. Those without private insurance will be able to access at-home tests through community sites such as health centers and rural clinics.

"For those not covered by private insurance, you're gonna make available free tests at thousands of convenient locations, locations for folks to pick them up and take a test kit home," Biden said Thursday. "Bottom line this winter, you'll be able to test for free in the comfort of your home and have some peace of mind."

The plan also aims to bolster outreach efforts to get more people vaccinated, ensure the equitable distribution of Covid -19 treatments to vulnerable populations and launch more than 60 rapid response teams to states to help combat the virus.

"The actions I'm announcing are ones that all Americans can rally behind, and should unite us in the fight against COVID-19," Biden said. "They come from a position of strength, we're in a better position than we were a year ago to fight Covid."

Last week, President Joe Biden ordered precautionary air travel restrictions for South Africa, Botswana, Zimbabwe, Namibia, Lesotho, Eswatini, Mozambique and Malawi. He did not indicate how long the bans, which took effect Monday, will remain in place. 

Those restrictions came into effect three weeks after the U.S. lifted a blanket ban on visitors from more than 30 countries, including the U.K., India, Brazil and South Africa, replacing them with vaccination requirements for tourists. Other countries implemented new travel restrictions last week, sending airline and other travel stocks tumbling.

The president on Monday also said his administration is working with vaccine manufacturers Pfizer, Moderna and Johnson & Johnson to develop contingency plans for additional vaccines or boosters tailored to protect against the new variant. 

He is directing the U.S. Food and Drug Administration and the CDC to use the "fastest process available without cutting any corners" to approve such potential vaccines. 

Current Covid-19 vaccines on the market are believed to provide some protection against omicron, Biden said Monday, adding that booster shots "strengthen that protection significantly." 

On Wednesday, White House chief medical advisor Dr. Anthony Fauci noted the profile of the variant suggests its mutations could reduce the effectiveness of current vaccines. However, more data is still needed, he added. 

Omicron has more than 30 mutations to the spike protein alone. Some of the mutations are associated with higher transmission and a decrease in antibody protection, according to the WHO. 

 — CNBC's Spencer Kimball and Leslie Josephs contributed to this report.