The Centers for Disease Control and Prevention confirms the first case of COVID-19 caused by the omicron variant in California.

The CDC says the person had recently returned from South Africa on Nov. 22, and is fully vaccinated.

Much is still unknown about the highly mutated omicron variant, including how contagious it is and whether it can evade vaccines. But governments have rushed to impose travel bans and other restrictions in hopes of containing it.

Nigeria and Saudi Arabia reported omicron infections Wednesday, marking the first known cases in West Africa and the Persian Gulf region.

Showing an increasingly complicated web of contagion, Japan reported an omicron case in a man who had come from Peru via Qatar.

European Union Commission President Ursula von der Leyen said it will take two to three weeks before it becomes fully clear what omicron can do to the world.

“This is, in normal times, a short period. In pandemic times, it’s an eternity,” she said.

South African researchers alerted the WHO to the omicron variant last week. It is not known where or when the variant first emerged, though it is clear it was circulating in Europe several days before that alert.

Nigeria initially stretched that timeline back even further, saying it found the variant in samples collected in October, but it later corrected that to say the cases were detected in travelers who arrived in the past week.

Many countries have barred travelers from southern Africa, and some have gone further. Japan has banned foreign visitors and asked international airlines to stop taking new reservations for all flights arriving in the country until the end of December.

On Wednesday, the WHO warned that blanket travel bans are complicating the sharing of lab samples from South Africa that could help scientists understand the new variant.

This is a developing story. Check back for updates. 

The Associated Press contributed to this report.

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