• Half of Americans are now fully vaccinated against Covid-19, a White House official said Friday.
• It's a major milestone as the nation battles a surge in new infections fueled by the delta variant.
• While the milestone is exciting, the country still has a long way to go before the pandemic is over, said Dr. Paul Offit, who advises the FDA on Covid vaccines.

Half of Americans, including all ages, are now fully vaccinated against Covid-19, a White House official said Friday, a major milestone as the nation battles a surge in new infections fueled by the delta variant.

More than 821,000 doses were administered over the previous day's total, including 565,000 people who got their first shot, White House Covid data director Cyrus Shahpar said in a tweet before the data was published on the Centers for Disease Control and Prevention's website. The seven-day average of new vaccinations is up 11% from last week and 44% over the past two weeks, he added.

While the milestone is exciting, the country still has a long way to go before the pandemic is over, said Dr. Paul Offit, who advises the Food and Drug Administration on Covid vaccines. The highly contagious delta variant continues to spread rapidly, particularly in regions of the nation with the lowest vaccination rates, he said.

"You had something like 100,000-plus cases and more than 600 deaths yesterday, which tells us that we're not there yet," Offit said.

The U.S. is reporting an average of about 98,500 daily infections, data compiled by Johns Hopkins University shows, already surpassing the peak in cases seen last summer when the nation did not have a vaccine.

Florida, Texas, Missouri, Arkansas, Louisiana, Alabama and Mississippi — states with some of the lowest vaccination rates — accounted for about half of new Covid cases and hospitalizations in the past week, Jeffrey Zients, the White House's Covid response coordinator, told reporters Thursday. Over the past seven days, 1-in-3 new Covid cases occurred in Florida and Texas.

In Florida, where Gov. Ron DeSantis is resisting calls for more restrictions, infection levels are nearing the state's pandemic peak in late January, when an average of nearly 18,000 new cases per day were being reported.

Florida is reporting an average of about 15,800 daily new cases over the past seven days, according to data compiled by Johns Hopkins University, up 51% from a week ago. That's the second-worst outbreak in the U.S. ranked by average daily new cases per capita, behind only Louisiana.

The death toll there is also on the rise at an average of 58 daily Covid deaths, up 45% from last week but below the record seven-day average of more than 180 daily deaths in late January.

The outbreak threatens to slow the nation's progress in ending the pandemic, especially as schools reopen and employers begin to bring workers back to the office this fall, health experts say.

With the virus circulating widely in states such as Florida, the nation is "likely to see even more worrying variants emerge this fall and winter," said Lawrence Gostin, director of the World Health Organization's Collaborating Center on National and Global Health Law.

"America is so close to getting back to normal. This could be a major setback to our national Covid response," he said.

As cases rise, more businesses are requiring their workers to get vaccinated, and federal health officials say they are urging states to offer incentives to their residents.

Some Americans are already seeing the impact of being unvaccinated and are now getting the shots, U.S. officials said Thursday.

In Arkansas, Mississippi, Louisiana and Alabama, the seven-day average of daily reported first doses has more than doubled since the start of July, CDC data shows, as the outbreak has worsened nationwide. In Arkansas, which is seeing the third-worst outbreak in the country based on daily new cases per capita, vaccinations have nearly tripled.

Zients said Thursday that the White House Covid Surge Response Teams are also working with 16 states that have rising cases to address their specific needs.

— CNBC's Nate Rattner contributed to this report.