Britain has found two cases of a coronavirus variant linked to South Africa, Health Secretary Matt Hancock said Wednesday, both of which are tied to contact with recent arrivals from that country. Hancock announced new restrictions on visitors from South Africa and called on anyone who has recently been to that country or been in contact with a recent arrival from there to quarantine immediately, describing the measures as temporary while officials seek to better understand the variant.

“This virus is highly concerning because it is yet more transmissible and appears to have mutated further than the new variant that’s been discovered in the U.K.,” he said at a news conference. South African officials announced last week that their scientists had detected a new variant that appeared to be fueling a rapid rise in infections there.

Experts have cautioned that both variants need additional study as scientists seek to better understand the mutations and what effect — if any — they will have on vaccines.

British authorities are tightening restrictions in response to the increase in cases. By Dec. 26, about 24 million people will face Britain’s toughest coronavirus rules, under which all nonessential businesses are required to close.

Many countries in Europe and elsewhere closed their borders to British travelers in recent days to try to limit the spread of the variant discovered in England. Some also placed restrictions on people traveling from South Africa.