LONDON — For the last four months, Britain has run a grand epidemiological experiment, lifting virtually all coronavirus restrictions, even in the face of a high daily rate of infections. Its leaders justified the approach on the grounds that the country’s rapid rollout of vaccines had weakened the link between infection and serious illness.

Now, with cases, hospital admissions and deaths all rising again; the effect of vaccines beginning to wear off; and winter looming, Britain’s strategy of learning to live with the virus is coming under its stiffest test yet.