The COVID-19 curve in the U.S. 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, lagging vaccination rates and Fourth of July 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. And all but two states Maine and South Dakota reported that case numbers have gone up over the past two weeks.

It is certainly no coincidence that we are looking at exactly the time that we would expect cases to be occurring after the July Fourth weekend, said Dr. Bill Powderly, co-director of the infectious-disease division at Washington Universitys School of Medicine in St. Louis.

At the same time, parts of the country are running up against deep vaccine resistance, while the highly contagious mutant version of the coronavirus that was first detected in India is accounting for an ever-larger share of infections.

Nationally, 55.6% of all Americans have received at least one COVID-19 shot, according to the Centers for Disease Control and Prevention. The five states with the biggest two-week jump in cases per capita all had lower vaccination rates: Missouri, 45.9%; Arkansas, 43%; Nevada, 50.9%; Louisiana, 39.2%; and Utah, 49.5%.

Even with the latest surge, cases in the U.S. are nowhere near their peak of a quarter-million per day in January. And deaths are running at under 260 per day on average after topping out at more than 3,400 over the winter a testament to how effectively the vaccine can prevent serious illness and death in those who happen to become infected.

Still, amid the rise, health authorities in places such as Los Angeles County and St. Louis are begging even immunized people to resume wearing masks in public. And Chicago officials announced Tuesday that unvaccinated travelers from Missouri and Arkansas must either quarantine for 10 days or have a negative COVID-19 test.