Prime Minister Boris Johnson of Britain once said his political hero was the mayor in the film “Jaws,” praising him for defying mass hysteria to keep the beaches open after a constituent is eaten by a shark.

As the coronavirus now stampedes across Britain and much of the world, Mr. Johnson is heeding the same principle, spurning the mass closures that have become commonplace across Europe and gambling his political future on a more restrained approach.

While countries across Europe have shut schools, sporting events and even restaurants and bars, Mr. Johnson has largely kept Britain open, opting for more targeted measures like asking people with respiratory symptoms to stay home. In effect, his government has said that mass closures will not halt the outbreak, and that exposing a large segment of the population will help build immunity and limit future infections.

That strategy has startled some epidemiologists, drawn criticism from a former health secretary and political ally, and prompted angry demands that Mr. Johnson’s government reveal more of its reasoning.