About six weeks ago, Erin Smith got the sinking feeling that a new surge of Covid-19 infections had hit New York City. The Covid-19 unit that she oversees at Manhattan’s Lenox Hill Hospital was full, with very sick patients, including some who died. It stayed that way for two to three weeks before almost emptying. Ms. Smith, a 39-year-old nurse manager, said she estimates it will be full again the first week of December.