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.
The upswing has been 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 case numbers had 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 University’s 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 Delta 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 Centres 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 US are nowhere near their peak of a quarter-million per day in January.
And deaths are running at fewer than 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 immunised people to resume wearing masks in public. And Chicago officials announced on Tuesday that unvaccinated travellers from Missouri and Arkansas must either quarantine for 10 days or have a negative Covid-19 test.
Meanwhile, the Health Department in Mississippi, which ranks dead last nationally for vaccinations, began blocking posts about Covid-19 on its Facebook page because of a “rise of misinformation” about the virus and the vaccine.
Mississippi officials are also recommending that people 65 and older and those with chronic underlying conditions stay away from large indoor gatherings because of a 150% rise in hospitalisations over the past three weeks.
In Louisiana, which also has one of the nation’s lowest vaccination rates, officials in New Orleans said they were likely to extend until autumn virus-mitigation efforts currently in place at large sporting and entertainment gatherings, including mask mandates or requirements that attendees be vaccinated or have a negative Covid-19 test.
State health officials said cases of the coronavirus were surging, largely among unvaccinated people.