Covid Spreads in Clusters. We Need to Track Them.

A fourth surge of Covid-19 is filling hospitals around the United States, fueled by the relaxing of mitigation measures, the more transmissible Delta variant and lagging vaccination rates. In states and counties where Delta is surging, contact tracers are overwhelmed. But monitoring the spread of the virus remains vital to getting it under control. The public health response to a Covid-19 outbreak in Provincetown, Mass., in July offers an object lesson in how Delta changes the playing field and what’s needed to manage the pandemic’s next phase