Early in the pandemic, contact tracing was viewed as one of the best options to quell the spread of coronavirus infections. The idea was to have public health workers track down people who tested positive, figure out whom they'd been in touch with and quickly get those people to quarantine. Places like Hong Kong and Singapore made headlines for their success stories. The U.S. aimed to replicate this, but came up short. Today, health reporter Selena Simmons-Duffin explains what went awry and the lessons learned.Reach the show by emailing shortwave@npr.org.