With each passing hour, new restrictions were being slapped on travel from countries in southern Africa as the world scurried Saturday to contain a new variant of the coronavirus that has the potential to be more resistant to the protection offered by vaccines.

A host of countries, including Australia, Brazil, Canada Iran, Japan, Thailand and the restrictions on travel, joined others, including the European Union and the U.K., in imposing restrictions on southern African countries in response to warnings over the transmissibility of the new variant — against the advice of the World Health Organization.

Despite the shutdown of flights, there was increasing evidence that the variant is already spreading. Britain became the latest country Saturday to report cases, with two confirmed. 

Cases have been reported in travelers in Belgium, Israel and Hong Kong, and Germany also has a probable case. Dutch authorities are checking for the new variant after 61 passengers on two flights from South Africa tested positive for COVID-19.

The global health body has named the new variant Omicron, labeling it a variant of concern because of its high number of mutations and some early evidence that it carries a higher degree of infection than other variants. That means people who contracted COVID-19 and recovered could be subject to catching it again. 

It could take weeks to know if current vaccines are less effective against it.

“It seems to spread rapidly,” U.S. President Joe Biden said Friday, a day after celebrating the resumption of Thanksgiving gatherings for millions of American families and the sense that normal life was coming back at least for the vaccinated. 

In announcing new travel restrictions, he told reporters, “I’ve decided that we’re going to be cautious.”

The variant’s swift spread among young people in South Africa — where the strain was first discovered — has alarmed health professionals even though there was no immediate indication whether the variant causes more severe disease. In just two weeks, Omicron has turned a period of low transmission in the country into one of rapid growth.

A number of pharmaceutical firms, including AstraZeneca, Moderna, Novavax and Pfizer, said they have plans in place to adapt their vaccines in light of the emergence of Omicron.

Amidst the heightened anxiety, there are voices of calm, like at the Texas Children’s Hospital Center for Vaccine Development, where Dr. Peter Hotez says there is no need to panic.

“As troubling as all the mutations in the spike protein are, the key is knowing how transmissible, and whether it has that ability to outcompete Delta. And that bar is high,” he said.

President Biden said the emergence of the new variant emphasizes the importance of being vaccinated and urged all Americans to get their booster shots.

“Every American who has not been vaccinated should be responsible and be vaccinated, from age five years and up, number one,” he said. “Number two: everyone eligible for the booster shot should get the booster shot immediately upon being eligible. That is the minimum that everyone should be doing.”

“We always talk about whether this is about freedom, but I think it’s a patriotic responsibility to do that,” he said.