The COVID-19 curve in the US is rising again after months of decline, with the number of new cases per day doubling over the past three weeks, driven by the fast-spreading Delta variant of the coronavirus, lagging vaccination rates and the July 4 Independence Day gatherings. Confirmed infections climbed to an average of about 23,600 a day on Monday, up from 11,300 on June 23, according to Johns Hopkins University data. All but two states – Maine and South Dakota – reported that case numbers have gone up over the past two weeks.