The team then projected what the new variant would do over the next six months and built models that factored in different levels of restrictions. Without a more substantial vaccine rollout, they warned, “cases, hospitalizations, I.C.U. admissions and deaths in 2021 may exceed those in 2020.”

Closing schools until February could buy Britain some time, the researchers found, but lifting those extra restrictions would then cause a major rebound of cases.

Because of the higher transmission rate, the country will need a much higher percentage of the population to get vaccinated to reach herd immunity. To reduce the peak burden on I.C.U.s, the researchers found, vaccination would need to jump to two million people per week from the current pace of 200,000.

“You need to be able to get whatever barriers to transmission you can out there as soon as possible,” Dr. Hanage said.

The researchers warned that their model was based, like any model, on a set of assumptions, some of which may turn out to be wrong. For instance, the rate at which infected people die from Covid-19 may continue to drop as doctors improve at caring for hospitalized patients. Uncertainties remain as to whether the new variant is more contagious in children, and if so, by how much.

Still, they wrote, “there is an urgent need to consider what new approaches may be required to sufficiently reduce the ongoing transmission of SARS-CoV-2.”

Alessandro Vespignani, director of the Network Science Institute at Northeastern University in Boston, who was not involved in the study, said of the new estimates, “Unfortunately, this is another twist in the plot.”