As countries around the globe scrambled to contain a new Covid-19 variant called omicron, scientists have said they are unsurprised at its emergence and repeated calls for greater worldwide vaccination efforts.

Two cases of the variant have been identified in the U.K., Health Minister Sajid Javid said Saturday. He added that the people involved were linked to each other and to travel to southern Africa, where omicron was first detected earlier this week.

Prime Minister Boris Johnson said Saturday anyone arriving in the UK will be asked to take a PCR test for Covid-19 on the second day and must self-isolate until they provide a negative test. He also said the rules on face coverings in shops and on public transport will be tightened.

Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, said” he “would not be surprised” if it had made its way to the U.S.

“We have not detected it yet,” Fauci said, “but when you have a virus that is showing this degree of transmissibility and you’re already having travel-related cases that they’ve noted in Israel and Belgium and other places when you have a virus like this, it almost invariably is ultimately going to go essentially all over.”

Identified as a “variant of concern” by the World Health Organization on Friday, the body said evidence suggested there was “an increased risk of reinfection” with the variant.

It was well known that viruses’ mutated and widespread vaccination was one of the most crucial ways of preventing this, several scientists Noted.

Covid “is still circulating quite extensively worldwide,” Rowland Kao, an epidemiologist at the University of Edinburgh, said, adding that he was “not really” surprised by the emergence of a new variant of concern. It was important for the world to exercise caution, but it was still unclear what impact omicron might have, he said.

“We’ve been saying all along that there was no reason to believe that delta was the last king of the variants,” Michael Osterholm, an epidemiologist at the University of Minnesota, said in a separate interview Friday.

He added that trying to contain the new variant could be like “trying to contain the wind,” as cases had already been identified in Belgium, Israel, and Hong Kong.

The U.S. has followed the lead of several other countries in ushering in new travel restrictions from South Africa and seven other countries starting Monday.

But Osterholm warned omicron already appeared to be “out of the barn.”

‘As long as it replicates, it will mutate’

All viruses, including SARS-CoV-2, the virus that causes Covid-19, evolve over time, and as a virus replicates, or makes copies of itself, small changes, or “mutations,” can occur, according to Deepti Gurdasani, a clinical epidemiologist at the U.K.’s the Queen Mary University of London.

When a virus spreads widely throughout a population, the chances of it mutating become higher, she added.

“As long as it replicates, it will mutate,” she said. “It’s like if you buy one lottery ticket, your chances of winning are low, but if you buy a million lottery tickets, it’s very likely you’ll win, and it’s the same with a virus.”

Most mutations are not a cause for concern, but if they affect a virus’s ability to spread or its virulence, meaning its ability to cause harm, the results can be dire, she added.

Worldwide access to vaccines ‘best hope’

Since the early days of the pandemic, the WHO and scientists around the world have called on the global community to do what it can to ensure all countries have access to vaccines to curb the global spread of the virus and prevent the development of new variants.

Worldwide access “offers the best hope for slowing the coronavirus pandemic, saving lives, and securing a global economic recovery,” the WHO’s website said. “There are enough doses of vaccines globally to drive down transmission and save many lives if they go to the people who need them most around the world,” it added.

Earlier this month, WHO Director-General Tedros Adhanom Ghebreyesus said countries that had immunized more than 40 percent of their populations should start focusing on donating doses to developing countries in need of vaccines.

“No more boosters should be administered except to immunocompromised people,” he said.

He also called on vaccine-makers to prioritize supplying jabs to COVAX, the WHO- and United Nations-backed effort to distribute vaccines globally.

It aims to distribute enough vaccines to provide protection to at least 20 percent of the population in 92 low- to medium-income countries, including South Africa, where omicron was first discovered.

The WHO has said it will take at least 11 billion doses to see at least 70 percent of the world’s population vaccinated.

But the program has seen nearly 548 million doses shipped to 144 countries, with a total of 5.59 billion doses secured, optioned, or received, according to data published by the United Nations.

Comparatively, more than 454 million doses have been administered in the U.S., where around 37.5 million people have already received a booster shot, according to data from the Centers for Disease Control and Prevention.

“Ideally, what you want is to have everybody vaccinated at the same time,” Kao said.

“The longer you drag that out, the more opportunities you have for mixing of vaccinated and unvaccinated people,” he said, adding that this would likely help further the spread of the virus and create more opportunities for mutations.

In September, the Biden administration announced the U.S. would be buying 500 million more doses of Pfizer’s Covid vaccine for the COVAX program, bringing the total number of shots the U.S. planned to donate to 1.1 billion.

President Joe Biden reiterated this message Friday.

“The news about this new variant should make it clearer than ever why this pandemic will not end until we have global vaccinations,” he said in a statement.

The U.S. had previously faced criticism from world leaders for rolling out booster shots before billions of others around the world could get their first doses.

However, the government has argued it can balance both, providing booster shots to its own population while also looking to help close the vaccine gap.