WASHINGTON — The Omicron coronavirus variant was reported in the United States for the first time on Wednesday in a traveler who had been in South Africa, as scientists around the world study whether the variant is more transmissible or virulent than its predecessors.
The patient, a resident of San Francisco, is in isolation, and aggressive contact tracing is underway, the Centers for Disease Control and Prevention said in a statement. The person was fully vaccinated, though without a booster shot, and was displaying mild symptoms that were improving, the agency said.
The discovery prompted the Biden administration to renew calls for everyone to get fully vaccinated and, if eligible, receive a booster. It also came as the C.D.C. asked airlines for the names and contact information of all passengers who had boarded flights bound for the United States since Nov. 29 and who had been in southern Africa in the past two weeks.
The World Health Organization has warned that the risk posed by the variant, which was first identified in South Africa on Thursday, is “very high.” More than 20 countries have detected the variant.
California health officials said the state was increasing coronavirus testing at airports, focusing on arrivals from countries identified by the C.D.C. as potential sources of the variant. Gov. Gavin Newsom said that California would not be intensifying public health restrictions, at least in the short term, but that “we should assume that it’s in other states as well.”
“There is no reason to panic, but we should remain vigilant,” he said in a statement. “The best thing we can do is to get vaccinated if you haven’t already, get your booster, and wear your mask indoors. As we continue to learn more about this variant, get tested if you have symptoms, and stay home if you’re sick.”
On Thursday, President Biden is expected to announce ways the government will ramp up its fight against the virus during the winter months, including tougher international travel restrictions and efforts to accelerate vaccine and booster availability.
Answers about whether Omicron is more contagious or deadly remain elusive as scientists around the globe race to map its attributes — including more mutations than the Delta variant — and seek to determine whether the vaccines will prove effective in protecting people from infection or serious hospitalization. Officials in South Africa have said that they do not know of any deaths associated with the variant, but health experts say it is too early to assess its true dangers.
Still, Omicron’s emergence during the holiday season, as Americans prepared to gather with relatives, raised the grim prospect of yet another surge in a pandemic that has severely tested the patience of a weary public, caused unparalleled economic damage and stoked political division.
Speaking to reporters shortly after the variant’s discovery was announced, Dr. Anthony S. Fauci, the president’s top medical adviser, expressed optimism that the United States would eventually emerge from the grip of the pandemic.
“This will end,” he said. “I promise you that this will end.”
Public health officials around the world have said for days that they expect the new, mutated form of the virus will quickly find its way to the United States despite the imposition of a travel ban by the Biden administration and other governments on international travelers from eight southern African nations late last month.
Scientific confirmation of the variant’s presence in the United States was nonetheless a jolt to Mr. Biden’s efforts to make good on his campaign promise to bring the pandemic to a swift and conclusive end. At the White House on Wednesday, he said that “we’re learning more every single day” and vowed that the administration would “fight this variant with science and speed, not chaos and confusion.”
Shortly afterward, Dr. Fauci told reporters that confirmation of the new variant in the United States should persuade unvaccinated Americans to get shots immediately.
“We have 60 million people in this country who are not vaccinated who are eligible to be vaccinated,” he said. “Let’s get them vaccinated. Let’s get the people vaccinated, boosted. Let’s get the children vaccinated.”
Dr. Fauci also urged caution, saying there was much that health officials did not know about the new variant.
“There’s a lot of information that is now evolving,” he said.
Scientists have said that Omicron carries more than 50 genetic mutations that in theory may make it more contagious and less vulnerable to the body’s immune defenses than previous variants. More than 30 of the mutations are in the virus’s spike, a protein on its surface. Vaccines train the body’s immune defenses to target and attack these spikes.
Available vaccines may still offer substantial protection against severe illness and death after infection with the variant, and federal officials are calling on vaccinated people to receive booster shots. The makers of the two most effective vaccines, Pfizer-BioNTech and Moderna, are preparing to reformulate their doses if necessary, but that will take time.
In California, Mr. Newsom said the infected person had not been hospitalized, and other people the individual came in contact with have not, at this point, tested positive. The governor said the patient traveled from South Africa, landed in the United States on Nov. 22 and started feeling mild symptoms on Nov. 25. The person was tested for the coronavirus on Nov. 28 and received a positive result a day later.
In a day, scientists at the University of California, San Francisco, had determined that it was the Omicron variant.
The person had received two doses of the Moderna vaccine but was within the six-month window of its effectiveness and had not received a booster, Mr. Newsom said.
Mayor London N. Breed of San Francisco said in a statement that the city had “one of the highest vaccination rates and lowest death rates in the country because of the actions our residents have taken from the beginning of this pandemic to keep each other safe.”
The city’s health director, Dr. Grant Colfax, added that “we are still learning about the Omicron variant, but we are not back to square one with this disease.”
About 79 percent of California residents have received at least one dose of a coronavirus vaccine. Cases and hospitalizations have largely been trending downward after a summer increase driven largely by the Delta variant.
In the Bay Area, where counties have some of the highest vaccination rates in the state, longstanding mask mandates had recently been relaxed or lifted as the spread of the virus slowed. Local governments in the state have begun to require businesses to verify vaccination status for entry, and more workers have been required to get their shots — a trend that officials have credited with helping to slow the spread.
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Throughout the pandemic, San Francisco County has avoided the crisis levels that affected some of the state’s other cities, like Los Angeles, as residents enthusiastically abided by restrictions, wore masks and got vaccinated. Seventy-seven percent of the county’s residents are fully vaccinated, according to its Department of Public Health.
Dr. Bob M. Wachter, the chairman of the department of medicine at U.C.S.F., said that the case would not be the last in California. But for now, he said, “I’m not changing my behavior any based on a single or a handful of cases.”
After news of the variant’s spread in South Africa, countries around the world curtailed air travel to and from the region — measures that officials there described as unduly punitive, especially in light of the fact that Western countries had failed to deliver sufficient vaccines and logistical support to the continent.
Still, the C.D.C. was considering plans to toughen requirements for screenings and coronavirus testing, such as asking travelers to provide a negative result taken within 24 hours before departure, among other steps.
Though the agency has yet to announce any changes, travelers were left scrambling for pre-emptive tests and reservation changes.
“It’s a shame, because travel just opened up again,” said Giritharan Sripathy, who was scheduled to fly to New York from London on Thursday.
Dutch officials said on Tuesday that they had identified cases of the variant a week before Friday, when 13 passengers who arrived on flights from South Africa tested positive for it, signaling that Omicron was already present in the Netherlands.
In South Africa, the variant accounts for most of the new daily cases reported in the nation’s most populous province, Gauteng, which is home to some 15 million people and the cities of Johannesburg and Pretoria.
At the White House on Wednesday, Dr. Fauci said he was “not so sure” that new testing requirements for international travelers, which administration officials are currently weighing, would have helped catch the case earlier. The patient had taken a test immediately after beginning to experience what he described as mild symptoms.
Dr. Fauci added that it was possible that the federal government could change its definition of “fully vaccinated” to require international travelers to have received booster shots before entering the United States.
Asked if Americans should feel free to attend holiday parties and drink beverages unmasked, he said it depended on the size of the gathering.
“In a situation with a holiday season, indoor-type settings with family that you know is vaccinated, people that you know, you can feel safe with not wearing a mask and having a dinner, having a reception,” Dr. Fauci said. But in larger public settings where it is unclear if everyone is vaccinated, he said, people should wear masks except to eat or drink.
Michael D. Shear reported from Washington, Shawn Hubler from Sacramento, and Roni Caryn Rabin from New York. Reporting was contributed by Jim Tankersley and Sheryl Gay Stolberg from Washington, Jill Cowan from Los Angeles, and Aina J. Khan from London.