Air China flight crew members in hazmat suits walk through the arrivals area at Los Angeles International Airport in Los Angeles, Tuesday, Nov. 30, 2021. Brazil and Japan joined the rapidly widening circle of countries to report cases of the omicron variant of the coronavirus on Tuesday.?(AP Photo/Jae C. Hong) 
A couple stand outside the security control section inside a terminal of the Barcelona Airport, Wednesday, Dec. 1, 2021. Health authorities in the Spanish capital have confirmed a second case of the omicron coronavirus variant in a 61-year-old woman who had returned from a trip to South Africa on Monday. Meanwhile lines have returned for those seeking vaccine shots in Portugal and Spain as they step up efforts to close the gap of the few residents still unvaccinated. (AP Photo/Joan Mateu Parra) 
Visitors wearing protective masks to help curb the spread of the coronavirus stroll at Sensoji Temple in the Asakusa district in Tokyo, on Tuesday, Nov. 30, 2021. The omicron variant kept a jittery world off-kilter as Japan further tightened travel restrictions, infections linked to the new version of the coronavirus popped up in more places and new evidence made clear the mutant strain was circulating weeks earlier than thought. (AP Photo/Koji Sasahara) 
Quarantine officers wait to guide travelers at the arrival hall of the Incheon International Airport In Incheon, South Korea, Wednesday, Dec. 1, 2021. South Korea’s daily jump in coronavirus infections exceeded 5,000 for the first time since the start of the pandemic, as a delta-driven surge also pushed hospitalizations and deaths to record highs. (AP Photo/Ahn Young-joon) 
People willing to be vaccinated sit in the vaccination booths at the Vaccination Center at Messe Dresden, Germany, Wednesday, Dec. 1, 2021. The vaccination centre, which was closed two months ago, has been restarted. (Sebastian Kahnert/dpa via AP) 
A photographer takes photo of a projector image showing immunofluorescence staining of omicron infected Vero E6 cells in Hong Kong, Wednesday, Dec. 1, 2021. Researchers at The University of Hong Kong have succeeded in isolating the omicron coronavirus variant from clinical specimens, making them the first research team in Asia that has succeeded in this regard, the university said. (AP Photo/Kin Cheung) 
A woman adds hair extensions to a woman’s hair, in an informal settlement in Zandspruit, South Africa, Wednesday Dec. 1, 2021. Despite the global worry, doctors in South Africa are reporting patients with the omicron variant are suffering mostly mild symptoms so far. But they warn that it is early. Also, most of the new cases are in people in their 20s and 30s, who generally do not get as sick from COVID-19 as older patients. (AP Photo/Jerome Delay) 
A health worker administers the vaccine for COVID-19 in Prayagraj, India, Wednesday, Dec. 1, 2021. ( AP Photo/Rajesh Kumar Singh) 
A nurse prepares vaccines in the Wizink Center, currently used for COVID-19 vaccinations in Madrid, Spain, Wednesday, Dec. 1, 2021. Health authorities in the Spanish capital have confirmed a second case of the omicron coronavirus variant in a 61-year-old woman who had returned from a trip to South Africa on Monday. (AP Photo/Paul White) 
Commuters wear protective face masks as they walk through a subway station, in Sao Paulo, Brazil, Wednesday, Dec. 1, 2021, amid the COVID-19 pandemic. Brazil joined the widening circle of countries to report cases of the omicron variant. (AP Photo/Andre Penner) 
People queue to be vaccinated against COVID-19 at the Lenasia South Hospital, near Johannesburg. Wednesday, Dec. 1, 2021. South African doctors say the rapid increase in COVID-19 cases attributed to the new omicron variant is resulting in mostly mild symptoms. (AP Photo/ Shiraaz Mohamed) 
A man receives Pfizer’s COVID-19 vaccine at a vaccination site in Marseille, southern France, Wednesday, Dec. 1, 2021. The new potentially more contagious omicron variant of the coronavirus popped up in more European countries on Saturday, just days after being identified in South Africa, leaving governments around the world scrambling to stop the spread. (AP Photo/Daniel Cole) 
A doctor speaks with children before administering them a COVID-19 vaccination, in Tulln, Austria, Wednesday, Dec. 1, 2021. Austria’s lockdown has officially been extended until Dec. 11 as planned amid signs that the measures are helping to bring down a sky-high coronavirus infection rate. A parliamentary committee signed off Tuesday on the extension of the country’s fourth national lockdown of the pandemic that started on Nov. 22. (AP Photo/Lisa Leutner) 
People wear mandatory face masks in a shopping street in Dortmund, Germany, Wednesday, Dec. 1, 2021. Germany tightened the COVID-19 rules because of the pandemic. (AP Photo/Martin Meissner) 
A Nigeria civil servant receives a dose of the AstraZeneca coronavirus vaccine, before she is allow access to her office in Abuja, Nigeria , Wednesday, Dec. 1, 2021. Nigeria has detected its first case of the omicron coronavirus variant in a sample it collected in October, weeks before South Africa alerted the world about the variant last week, the country’s national public health institute said Wednesday. (AP Photo/Gbemiga Olamikan) 
People line up to be vaccinated against COVID-19 in Lawley, south of Johannesburg, South Africa, Wednesday, Dec. 1, 1021. South African doctors say the rapid increase in COVID-19 cases attributed to the new omicron variant is resulting in mostly mild symptoms. (AP Photo/ Shiraaz Mohamed)
SAN FRANCISCO (AP) — The U.S. recorded its first confirmed case of the omicron variant Wednesday — in a vaccinated traveler who returned to California after a trip to South Africa — as scientists around the world race to establish whether the new, mutant version of the coronavirus is more dangerous than previous ones.
Dr. Anthony Fauci, the United States’ top infectious-disease expert, made the announcement at the White House.
“We knew it was just a matter of time before the first case of omicron would be detected in the United States,” he said.
The infected person was identified as a traveler who had returned from South Africa on Nov. 22, developed mild symptoms and tested positive for COVID-19 Monday. Researchers at the University of California, San Francisco obtained a sample from the patient Tuesday evening and worked feverishly overnight to assemble the genetic sequence.
The person, who had had the full two doses of the Moderna vaccine and wasn’t yet due for a booster shot, is improving, California officials said.
Fauci and other medical experts strongly emphasized that Americans should continue to get vaccinated and get their booster shots. The vaccine has been proven to reduce the risk of severe illness and death, and Fauci said it is reasonable to believe it will offer protection against the omicron variant.
The mild nature of the California case “is a testimony to the importance of the vaccinations,” said California Health Secretary Dr. Mark Ghaly.
All the individual’s close contacts have been reached and have tested negative, officials said. The patient, who agreed to remain in quarantine, was identified only as being between 18 and 49.
California Gov. Gavin Newsom expressed confidence in the state’s efforts to control the virus and said he does not anticipate it will impose another stay-at-home order or other shutdown measures.
At least 23 other countries have reported omicron infections since South African authorities first identified the variant a week ago — an announcement that led the U.S. and many other countries to almost immediately bar airline travelers arriving from southern Africa.
In South Africa, new cases of COVID-19 nearly doubled in a single day to almost 8,600, authorities reported Wednesday, and the country’s National Institute for Communicable Diseases said omicron has now overtaken the delta variant among genetic samples sequenced.
But the variant is still surrounded by many unknowns, among them: Is it more contagious than other versions, as some scientists are beginning to suspect? Does it make people more seriously ill? And can it evade the vaccine?
“Any declaration of what will or will not happen with this variant, I think it is too early to say,” Fauci said.
Genomic sequencing on the patient’s virus from UCSF was confirmed by the Centers for Disease Control and Prevention.
“We will likely see this scenario play out multiple times across the country in the coming days or weeks,” said Scott Becker, CEO of the Association of Public Health Laboratories.
“This particular case shows the system working as it was designed to work — an individual with travel history from South Africa, an astute laboratory and quick prioritization of the specimen for sequencing, and close coordination with public health officials.”
Nigeria and Saudi Arabia also reported omicron infections Wednesday, marking the first known cases in West Africa and the Persian Gulf region.
It is not known precisely where or when the variant first emerged, though it is clear it was circulating in Europe several days before South Africa sounded the alarm.
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 lamented.
At the same time the omicron variant is spreading new fear and uncertainty, the dominant delta variant is still creating havoc, especially in Europe, where many countries are dealing with a surge in infections and hospitalizations and some are considering making vaccinations mandatory.
Going further than many other countries in trying to contain the virus, 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.
The U.S. is working toward requiring that all air travelers to the country be tested for COVID-19 within a day before boarding their flights, up from the current three days.
On Wednesday, the World Health Organization warned that blanket travel bans are complicating the sharing of lab samples from South Africa that could help scientists understand the new variant.
World leaders continued to emphasize that the best way to contain the pandemic remains vaccinations.
For the first time, von der Leyen said EU nations should consider making vaccinations mandatory, as several have done for certain sectors, or as Austria has done overall. Altogether, 67% of the EU’s population is vaccinated, but that relatively high rate hasn’t stopped several countries from seeing surges.
Greece plans to impose fines of 100 euros ($113) per month on people over 60 who don’t get vaccinated. Slovakia is considering giving that age group 500 euros ($565) if they step forward for the shot. German Chancellor-designate Olaf Scholz, meanwhile, said he will back a proposal to mandate vaccinations for everybody.
By ZEKE MILLER, OLGA R. RODRIGUEZ and MICHAEL BALSAMO