Around the world, the pandemic is putting the time-honored public health technique to a grueling new test. And the stakes are particularly clear to the U.S. city that has suffered more COVID-19 deaths than any other but wrestled its outbreak into relative control late this spring. The city has hired over 3,000 tracers, and Mayor Bill de Blasio, a Democrat, recently credited the effort with "so far, amazing success." After a knotty June start, there have been some changes — such as asking people how they're feeling before asking their demographic particulars — and the city says it's now meeting its goal of reaching about 90% of all newly diagnosed people and completing interviews with 75%. But in the program's first two months, more than 11,000 infected people didn't provide any names of others they might have exposed. Only 6 in 10 of the contacts who are named complete interviews, short of the city's goal. The city hasn't said how quickly it's connecting with people or what it's gleaning about potential sources of exposure. Officials say they're analyzing that data. New York's numbers compare fairly favorably with at least some other state and local programs'; reporting is patchy. Still, some outside experts suggest New York should drill down and disclose more on its effects, not just its activity, and on clues the data could yield about squelching the virus' spread.