Countries are bracing for the impact of more contagious virus variants.

Many countries expect that B.1.1.7’s impact still lies ahead.

That is a disturbing possibility in the United States, which has long had the world’s largest coronavirus outbreak and is in the midst of a post-holiday surge. On Friday, federal health experts warned in dire terms that B.1.1.7 would most likely be the dominant source of infection in the country by March.

Nearly 20 European countries have found B.1.1.7 so far. In Denmark on Saturday, the authorities said more than 250 cases had been detected in samples taken since November. The country’s health minister has predicted that the variant will predominate by mid-February. The country’s coronavirus monitor also reported that it had identified a case of the variant found in South Africa, Reuters reported.

Many countries in Europe are redoubling their efforts at mitigation. A nationwide 6 p.m. curfew went into effect in France on Saturday, and the authorities have warned that they could reimpose strict lockdown measures. Scotland tightened already strict restrictions, including banning drinking outside and barring customers from stepping inside establishments to buy takeaway food or coffee. Britain and Germany have closed schools.

In a stark contrast, the authorities in Spain have refused to impose a new nationwide lockdown, arguing that the recent discovery of dozens of cases of the variant were not to blame for a record surge in infections.

On Saturday, Britain reported eight cases of one of the variants found in Brazil, hours after British authorities imposed a travel ban from Latin American countries and Portugal, which is tied to Brazil by its colonial history and by current travel and trade ties. Italy also suspended flights from Brazil, its health minister, Roberto Speranza, announced on Facebook.

A leading epidemiologist said that a second variant discovered in Brazil was most likely already present in Britain.

“We are one of the most connected countries in the world, so I would find it unusual if we hadn’t imported some cases into the U.K.,” Professor John Edmunds, a member of a group of scientists advising the government on the pandemic, said about the second variant, which was found in the Brazilian city of Manaus.