Rich Eisen of the NFL Network announced Thursday he has tested positive for COVID-19, despite being vaccinated.

“Double-vaxxed. COVID+ It’s possible, folks,” Eisen wrote. “As someone sitting Day 4 in quarantine fighting off symptoms, I can personally attest you still need to be careful and, most importantly, get vaccinated. Why? Especially since mine didn’t keep COVID from my body?”

Eisen, 52, said he received two shots of Pfizer back in February. According to health care professionals that he’s crossed paths with over the last few days, his symptoms could have been worse if he didn’t have the vaccine.

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.

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 Atlanta-based 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.

Meanwhile, the Health Department in Mississippi, which ranks 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 hospitalizations over the past three weeks.

The Atlanta Journal-Constitution speaks with Dr. David Kessler the White House vaccine chief about the Covid-19 Delta variant.

In Louisiana, which also has one of the nation’s lowest vaccination rates, New Orleans officials said Tuesday they are likely to extend until fall 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 are surging, largely among nonvaccinated people.

But the political will may not be there in many states fatigued by months of restrictions.

In Michigan, Democratic Gov. Gretchen Whitmer is facing a drive to repeal a law that she used to set major restrictions during the early stages of the pandemic.

And Republican Gov. Kay Ivey of Alabama pushed back against the idea that the state might need to reimpose preventive measures as vaccinations lag and hospitalizations rise.

“Alabama is OPEN for business. Vaccines are readily available, and I encourage folks to get one. The state of emergency and health orders have expired. We are moving forward,” she said on social media.

Dr. James Lawler, a leader of the Global Center for Health Security at the University of Nebraska Medical Center in Omaha, said bringing back masks and limiting gatherings would help. But he acknowledged that most of the places seeing higher rates of the virus “are exactly the areas of the country that don’t want to do any of these things.”

Lawler warned that what is happening in Britain is a preview of what’s to come in the U.S.

“The descriptions from regions of the world where the delta variant has taken hold and become the predominant virus are pictures of ICUs full of 30-year-olds. That’s what the critical care doctors describe and that’s what’s coming to the U.S.,” he said.

He added: “I think people have no clue what’s about to hit us.”

The Associated Press contributed to this report.