Over the Thanksgiving weekend, a new variant took hold.

Omicron, first reported by scientists in South Africa on Thursday, has been reported in at least ten countries, including Britain, Israel, and Australia. Its mutations have many worried that it could be more transmissible, could cause more severe disease, or could evade vaccines more easily than other variants. The World Health Organization named it a "variant of concern" on Friday.

However, President Biden said Monday that the new variant should be "a cause for concern, not a cause for panic," urging people once again to get vaccinated, including booster doses.

But there's still a lot we're waiting to learn about Omicron.

Here's what we know:

• The best thing you can continue to do is get vaccinated, get your booster dose, and wear your mask.
• Biden is not currently expecting the need for a widespread lockdown of the country.
• CDC Director Rochelle Walensky told reporters today the agency is considering tighter travel restrictions (more on current US travel restrictions below).

And here's what we don't know:

• The most significant unknown is how effective current vaccines will be against Omicron. All three manufacturers of the US-approved COVID-19 vaccines plan to test their vaccines against Omicron, but data won't be available for a few weeks. Moderna Chairman Noubar Afeyan said today that the company's three-shot regimen may protect against the variant.
• It's also not yet clear whether Omicron will cause more severe cases of COVID-19. The initial reports of cases in South Africa came from a group of university students, so researchers have warned the mild nature of the cases there could be distorted by the age of the patients.

US health officials are expanding a surveillance program to try to figure out whether Omicron has entered the United States yet.