China’s quarantine of more than 35 million people, almost certainly the largest in modern public-health history, is surprising and troubling experts who said such drastic restrictions rarely work and often backfire. In the United States, mandatory limits on movement for people in whole cities or regions have received little serious consideration in planning for disease outbreaks like the coronavirus infection now sweeping across China,according to public-health authorities and a review of government reports.

“This is just mind-boggling: This is the mother of all quarantines,” said University of Michigan medical historian Howard Markel. “I could never have imagined it.” Cordoning off a region like China’s Hubei province pens the sick together with the uninfected. It increases the burden on authorities, who must ensure the flow of food, water and other supplies to the quarantined area. It is nearly impossible to enforce. And it sows distrust in government at a time when public support is essential, prompting people to evade the restrictions or refuse to report their symptoms, experts said.