In trying to forecast what lies ahead, American health officials and scientists have debated what mixture of factors has driven up case numbers in Europe and just how serious the wave there could get.

The BA.2 subvariant began its march across Europe around the time that certain countries were lifting restrictions and mask mandates, potentially giving it extra kindling for its spread. Some scientists in Britain have also attributed that country’s surge in part to the fact that immunity tends to weaken over time following vaccinations or earlier infections.

In some parts of Europe, like Denmark and the Netherlands, the peak of the BA.2 wave has already passed. In other countries, where case numbers have climbed since early March, hospital admissions have remained flat or risen only slightly.

But Britain has emerged as a more startling example of the potential for a surge in BA.2 cases to begin filling up hospital beds, too. People 70 and older in England have been infected at record levels, health officials said, with estimates that roughly one in every 30 people in that age group had Covid in the week before March 12.