With concerns about the recently discovered omicron variant of COVID-19, there will be stricter testing requirements for all travelers entering the U.S.

Starting Monday Dec. 6, all inbound international travelers will be required to test within one day before boarding flights to the U.S. – regardless of their vaccination status or country of departure. Currently, vaccinated international travelers can present a negative result obtained within three days.

On Dec. 6, all flights leaving after 12:01 a.m. EST will need to abide by this new testing order, which is now posted on the CDC website.

President Biden also announced an extension for the federal mask mandate that requires travelers to wear masks in airports, on planes, and on modes of public transportation such as trains and buses. This mandate has been extended through March 18.

Currently, there is not a post-arrival testing or quarantine requirement for arriving travelers. According to report, officials have been looking at a requirement that all travelers get retested within three to five days of arrival. Additionally, officials are considering a proposal that mandates all travelers to self-quarantine for seven days, even if their test results are negative.

The omicron variant was first identified in South Africa. As of Wednesday, the U.S. joins more than 20 countries that have found at least one case of the strain since South African health officials made the announcement on Nov. 25.

Along with several other countries, the U.S. has banned the arrival of non-citizens who were physically present within these eight African countries during the 14-day period preceding their entry into the U.S.: Botswana, Eswatini, Lesotho, Malawi, Mozambique, Namibia, South Africa, and Zimbabwe, according to the CDC. Countries like Japan and Israel have currently banned all foreign travelers from entering their borders.

The World Health Organization (WHO) has classified the new omicron variant as a variant of concern due to its large number of mutations. However, it could be several weeks before scientists can say whether the omicron variant might be able to elude current vaccines and treatments.