In Arizona’s most populated region, the coronavirus is so ubiquitous that contact tracers have been unable to reach a fraction of those infected.

In Austin, Tex., the story is much the same. Just as it is in North Carolina, where the state’s health secretary recently told state lawmakers that its tracking program was hiring outside workers to keep up with a steady rise in cases, as a number of other states have done.

Cities in Florida, another state where Covid-19 cases are surging, have largely given up on tracking cases. Things are equally dismal in California. And in New York City’s tracing program, workers complained of crippling communication and training problems.

Contact tracing, a cornerstone of the public health arsenal to tamp down the coronavirus across the world, has largely failed in the United States; the virus’s pervasiveness and major lags in testing have rendered the system almost pointless.