U.K. extends boosters to all adults while announcing two new Omicron cases.

Britain expanded its vaccine booster program to all adults on Monday, stepping up its response to the newly discovered Omicron variant of the coronavirus.

The government also announced two new cases of the variant in England, just hours after Scotland said that six cases had been detected there and that contact tracing was being conducted. Nationally, Britain has identified 11 cases.

Over the weekend, Prime Minister Boris Johnson announced new mask mandates and testing requirements for travelers to Britain. While the government has not ordered people to work from home where possible, or mandated the use of vaccine passports or masks in English restaurants, officials have not ruled out the possibility.

Jonathan Van-Tam, Britain’s deputy chief medical officer, said that while there was still a high level of uncertainty about the variant, the country would expand the vaccine program right away.

“We don’t know what’s going to happen next,” Mr. Van-Tam said, noting that it could take scientists weeks to better understand the variant. “But whilst we wait for the mist to clear on what this concerning variant actually means, there is no time to delay. It’s our opportunity to get ahead, and vaccine boosting is the thing we can do most effectively while we wait for that mist to clear.”

The British government was widely criticized for a sluggish response to the Delta variant earlier this year, and its reaction to the Omicron variant came markedly quicker.

Britain’s vaccine advisory board, the Joint Committee on Vaccination and Immunization, advised everyone ages 18 to 39 to get a booster shot; previously, people 40 and over were eligible. It reduced the required waiting period between the initial vaccine series and the booster from five months to three.

The board also said children ages 12 to 15 could receive a booster shot and recommended that those who are severely immunocompromised receive a fourth dose.

The Education Department has advised students in England ages 11 and up to wear face masks in communal areas beginning Monday.

Nicola Sturgeon, the first minister of Scotland, said she and the leader of Wales, Mark Drakeford, had written to Mr. Johnson to demand that all travelers to Britain be required to take a coronavirus test on the second and eighth day after their arrival, and that they be required to isolate for that whole period. Under the most recent guidance, arrivals will only have to take a test on the second day.

Ms. Sturgeon and Mr. Drakeford have also called for a joint meeting of the British government’s top emergency committee, Cobra, to better coordinate the response to the new variant.

Unlike many countries in Europe, Britain has had relatively few restrictions in place since the summer, and the government has repeatedly said there are no plans for another lockdown.

Speaking in front of Parliament on Monday, the British health secretary, Sajid Javid, reinforced that philosophy. If Omicron proved to be “no more dangerous” than the Delta variant currently dominant in Britain, he said, then “we wouldn’t keep measures in place for a day longer than necessary.”