A new, possibly more infectious coronavirus variant, with an unusual number of mutations, had scientists sounding the alarm Friday and countries including the United States moving to impose travel restrictions as the world feared another setback on the long road out of the pandemic.

Senior Biden administration officials announced that starting Monday, travel to the United States will be restricted from South Africa and seven other countries — Botswana, Eswatini, Lesotho, Malawi, Mozambique, Namibia and Zimbabwe. The new policy, which does not affect U.S. citizens and permanent residents, was enacted “out of an abundance of caution,” officials said, as scientists work to learn more about the new variant.

This latest variant arrives nearly two years after the first ominous warnings about a novel coronavirus that emerged from China. Now, even as the world grows increasingly weary of measures to contain the virus, this new threat is casting a shadow across holiday celebrations.

On Friday, the World Health Organization declared the mutation-laden virus a “variant of concern” after a full-day review by the group’s scientists and dubbed it the “omicron” variant, named for a Greek letter.

“This variant has a large number of mutations, some of which are concerning,” the WHO said in a statement Friday. “Preliminary evidence suggests an increased risk of reinfection with this variant, as compared to other” variants of concern.

In many ways, the virus is acting exactly as global health experts have warned it might — creating new and potentially more dangerous variants, first detected in undervaccinated parts of the world. Still, major questions remain about omicron’s origins, transmissibility, whether it might make people sicker and whether it might be able to evade vaccines or therapeutics.

“It’s too early to know the effectiveness of current vaccines against the new variant,” said Bruce Gellin, a vaccine expert and chief of global public health strategy at the Rockefeller Foundation.

Scientists in South Africa convened a news conference Thursday and said they had linked omicron to an exponential rise of infections in the country. Cases have also been identified in Belgium, Botswana, Hong Kong, Israel and other countries.

Financial markets plummeted amid news of the variant, with the Dow Jones industrial average delivering its worst day of 2021 with a 905-point plunge. By the time trading stopped at 1 p.m. because of the holiday weekend, the Dow had shed about 2.5 percent, the S&P 500 had erased 2.3 percent and the tech-heavy Nasdaq had declined 2.2 percent.

The World Trade Organization postponed its first ministerial meeting in four years because of the deteriorating health situation, Reuters reported.

Britain, Canada, France, Israel and Japan began to ban or order quarantines for air passengers arriving from the southern African region. European Union countries agreed Friday to ban air travel from seven southern African countries.

“Our view is very clear,” Dana Spinant, deputy chief spokeswoman for the European Commission, said at a news conference. “We need to act very fast, we need to be vigilant, and we need to take all measures that are appropriate at this stage to prevent this virus from entering Europe.”

Chief among experts’ concerns is whether coronavirus vaccines will prove less powerful against omicron — and whether they could be adapted to target the new variant.

In a statement, Pfizer and BioNTech said the companies expect to have data within two weeks that will provide more information on whether omicron may require an adjustment to the companies’ coronavirus vaccine.

Pfizer and BioNTech have been working to adapt their mRNA vaccine “within 6 weeks and ship initial batches within 100 days in the event of an escape variant.” The companies have begun clinical trials to test their vaccine against other variants that could provide blueprint studies for regulators if a variant-specific vaccine is needed.

Moderna, maker of another mRNA coronavirus vaccine, said Friday it was “working rapidly” to test the ability of its vaccine to neutralize the omicron variant and that it expected to have data “in the coming weeks.” It also plans to advance an omicron-specific booster candidate, according to a company statement.

The omicron variant, Moderna said, carries a combination of mutations that poses a “significant potential risk” to both natural and vaccine-induced immunity.

Gellin said U.S. regulators have “not yet mapped the process they would use to make adjustments to coronavirus vaccines.” But small clinical trials would probably be needed to ensure that changes had the effect desired.

The Food and Drug Administration is familiar with updating vaccines, Gellin said, and has a long track record of approving flu vaccines to “match” the viruses in circulation every year.

“There is no reason why future boosters should not be updated to match the most common strain” of the coronavirus, said John Brownstein, chief innovation officer at Boston Children’s Hospital.

Africa began ramping up its monitoring of variants several months ago, made wary by the delta variant’s destructive sweep across India and the continent’s low vaccination rates, which some officials blamed on “vaccine hoarding” by wealthier countries. In South Africa, just 25 percent of the population is vaccinated, and officials said the vast majority of people hospitalized continue to be unvaccinated.

Biden administration officials said Friday the United States has shipped more than 275 million doses of coronavirus vaccine to 110 countries, including 93.9 million for Africa, and 13.3 million to the newly restricted countries, and that it has worked to boost vaccine manufacturing in South Africa.

Top South African health officials said they began exploring the possibility of a new variant when they discovered a fast-growing cluster of cases in the nation’s populous Gauteng province. The officials said they wanted to announce the new variant to help prevent further spread — and scientists around the world noted that the variant was detected far more quickly than the delta variant, which was identified in India and quickly spread around the world.

But South African health officials decried the world’s reaction as unjustified, saying South Africa should not be “punished” for making this discovery, particularly because the variant’s origin remains unknown.

“This kind of knee-jerk reaction really does not make sense,” said Joe Phaahla, South Africa’s health minister. “Many of these countries that are coming with this draconian reaction are battling their own fourth wave.”

“The world should provide support to South Africa and Africa and not discriminate or isolate it!” tweeted Tulio de Oliveira, director of South Africa’s Center for Epidemic Response and Innovation, calling on philanthropists to help. “By protecting its poor and oppressed population we will protect the world,” de Oliveira wrote.

The European Center for Disease Prevention and Control offered a new threat assessment Friday noting the distinct possibility that the new variant will spread.

Even before the latest variant emerged, Europe was applying new lockdowns and travel restrictions for unvaccinated residents in an attempt to contain a surge in coronavirus cases. Reported deaths in Europe reached nearly 4,200 a day last week — twice the number since the end of September, according to the World Health Organization, which counts 53 countries as part of its European region. Covid-19 cases fueled by the delta variant have been surging in some U.S. states, too, including Michigan and Minnesota.

“We just don’t know,” said Brownstein, who emphasized the importance of waiting for research.

“Those mutations are worrisome,” said Albert Ko, an infectious-disease epidemiologist at the Yale School of Public Health, who said they may compromise treatments known as monoclonal antibodies. “That’s the proof that has to be done,” said Ko, who pointed out that multiple mutations can also cripple a virus, making it less fit.

“Not all mutations are good for the virus,” Ko said.

Massimo Ciccozzi, head of the molecular epidemiology unit at the Rome-based University Campus Bio-Medico, described omicron as “a greatest hits compilation of all mutations from other variants, within one lineage.”

Ciccozzi, who speculated that the package of mutations may be due to long-term treatment of a single immunocompromised patient, noted one bright spot. Unlike delta, omicron is detectable through the normal procedures used with other variants.

“The molecular test detects it,” Ciccozzi said. “Which is very important because it just won’t go unnoticed.”

Susan Hopkins, chief medical adviser to Britain’s Health Security Agency, told BBC Radio that South Africa had a very low level of cases at one point and then “in a shorter period than two weeks, they have more than doubled their epidemiology picture.”

Britain maintains a world-class system of laboratories to detect and study the genomes of coronavirus variants and modeling groups that forecast potential spread. The southern African strain has not been found in Britain.

Israel imposed a travel ban covering most of Africa after the detection of the new variant.

“We are currently on the verge of a state of emergency,” Israeli Prime Minister Naftali Bennett said.

In Japan, travelers arriving from six African countries will need to quarantine at designated accommodation facilities for 10 days beginning Saturday, rather than quarantining at home, Chief Cabinet Secretary Hirokazu Matsuno announced Friday evening.

India on Thursday alerted all states to the new variant, advising them to test and screen everyone traveling from and transiting through the “at-risk” countries of South Africa and Botswana, as well as Hong Kong. India earlier this month opened its borders to vaccinated international travelers after more than a year and a half.

“What’s interesting is the policies,” said Ko, the Yale epidemiologist, noting how the coronavirus has consistently beaten global responses. “Are any of these policies going to help?”

Joel Achenbach, William Branigin, Erin Cunningham and Aaron Gregg in Washington; Quentin Ariès in Brussels; Amanda Coletta in Toronto; Michelle Ye Hee Lee in Seoul; Niha Masih in New Delhi; Loveday Morris in Berlin; Stefano Pitrelli in Rome; and Shira Rubin in Tel Aviv contributed to this report.