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President Joe Biden is set to deliver remarks Monday about the new Covid variant omicron, the heavily mutated strain of the coronavirus that has drawn concern from markets and governments around the world since its discovery less than three weeks earlier.

The White House on Friday ordered precautionary air travel restrictions from South Africa, where omicron was first detected, and seven other African countries. Those new rules, which took effect Monday, were announced after the Dow Jones industrial average suffered its deepest rout of the year, driven by fears of another deadly wave of the pandemic.

But much remains unclear about the threat posed by the new variant. Scientists, including top U.S. infectious disease expert Dr. Anthony Fauci, say it may take weeks to understand omicron’s transmissibility, the severity of its symptoms and the effectiveness of existing vaccines against it.

Biden, speaking from the White House, is expected to reiterate his calls for Americans and others around the globe to get vaccinated and seek out booster shots for Covid as the best way to protect themselves and others from the pandemic.

“I’ve decided we’re going to be cautious,” Biden told reporters Friday after announcing the new travel rules. “We don’t know a lot about the variant except it is a great concern, seems to spread rapidly,” he said.

The U.S. is still working to contain the highly contagious delta variant that scuppered Biden’s hopes of moving past the pandemic in 2021. That variant is now by far the most prevalent in the nation.

Read CNBC’s live updates to see the latest news on the Covid-19 outbreak.