Top Biden administration officials on Thursday said that a hyper-transmissible variant of the coronavirus is posing new challenges for the nation’s health system, urging millions of unvaccinated Americans to get shots to protect themselves and their communities.

The delta variant, first detected in India, now represents more than 83 percent of cases circulating in the United States, according to the Centers for Disease Control and Prevention. People infected with the variant appear to carry a viral load that is more than 1,000 times that of those infected with earlier forms of the virus, allowing the virus to spread rapidly among unvaccinated people, scientists have found.

“The delta variant is more aggressive and much more transmissible than previously circulating strains. It is one of the most infectious respiratory viruses we know of and that I have seen in my 20-year career,” CDC Director Rochelle Walensky told reporters.

“We are yet at another pivotal moment in this pandemic, with cases rising again and some hospitals reaching their capacity in some areas,” Walensky added. “We need to come together as one nation, unified in our resolve to protect the health of ourselves, our children, our community, our country and our future with the tools we have available.”

The daily average of confirmed coronavirus cases has roughly quadrupled during July, from about 13,000 per day at the start of the month to 43,243 now, according to The Washington Post’s seven-day average of infections. The virus is spreading most rapidly across the South and Midwest, in states with low vaccination rates, and hospital officials there say they are reeling from a new surge of patients, driven by the delta variant.

For instance, coronavirus-related hospitalizations in Alabama have more than doubled this month, with 213 patients in intensive care units, up from 79 on July 1, according to The Post’s tracking. Only 34 percent of Alabama residents have been fully vaccinated against the virus.

“If you have not been vaccinated for #COVID19, now is the time,” the Alabama Hospital Association exhorted on Twitter on Tuesday. “Cases are on the rise and the vaccine could save your life.”

The White House said that it is moving to respond to the groundswell of infections, announcing an additional $1.6 billion in funding to boost coronavirus testing and prevention in settings where people live in proximity, such as prisons and homeless shelters. NBC News first reported the funding.

Biden officials also announced $100 million in funding for rural health clinics in communities with low vaccination uptake, seeking to boost education and outreach, and detailed their efforts to deploy surge response teams to areas seeing virus spikes, including Missouri and Nevada.

“We know everyone’s vaccination journey is different. We are ready to get more Americans vaccinated whenever, wherever they’re ready,” said Jeff Zients, the White House coronavirus coordinator.

Officials declined to answer questions about whether the White House is considering new masking messages in light of the delta variant’s spread, reiterating that the CDC would decide whether to revise its guidance. The Post reported Wednesday that senior officials are debating whether they should urge vaccinated Americans to wear masks in more indoor settings.

Public health experts said they have growing questions about the risk of vaccinated people contracting the virus and infecting others, particularly because the CDC is tracking only the most serious of those cases.

“There are clearly examples of vaccinated people spreading the virus. The question is, how often is it happening? We don’t know the answer to that,” said Walid Gellad, the director of the University of Pittsburgh’s Center for Pharmaceutical Policy and Prescribing.

Gellad criticized the CDC for not offering additional guidance for Americans who have been fully vaccinated, such as when they should be getting tested.

“Preparing the public and preparing clinicians for breakthrough infections, that’s the piece I think we’re not seeing,” Gellad said. “They need to readdress that. We need to know these things now, because two months from now is too late.”

Officials in Texas, New Jersey and other states have said that dozens of vaccinated, often elderly Americans have died of covid-19, but cautioned that that is a very small percentage of overall deaths linked to the virus.

Senior administration officials said that the current vaccines remain highly effective against the delta variant.

“It's important to remember … infections after vaccination are expected,” infectious-disease expert Anthony S. Fauci told reporters. “No vaccine is 100 percent effective. However, even if a vaccine does not completely protect against infection, it usually, if it's successful, protects against serious disease.”

A full course of the Pfizer-BioNTech vaccine is slightly less effective against the delta variant than it was against a previously dominant version of the virus, according to a study published in the New England Journal of Medicine on Thursday. Researchers found that two doses of the vaccine offered 88 percent protection against symptomatic disease caused by the delta variant, compared with 94 percent against the earlier alpha strain of the virus that was predominant at the beginning of the year.

In recent days, a growing number of Republicans have urged their supporters to get vaccinated, citing the threat of the delta variant and noting that millions of conservative voters say they don’t want the vaccine. “I just felt the time was right to do it,” Rep. Steve Scalise (La.), the no. 2 House Republican, told The Post in an interview Tuesday.

But other Republicans have said the rising fears are overblown.

“There’s so much optimism that should be out there right now,” Sen. Roger Marshall (Kan.) said on Fox Business Network, acknowledging the delta variant is “spreading a little bit across the country.”

Biden officials sounded a more sober note.

“If you are not vaccinated, please take the delta variant seriously,” Walensky said. “This virus has no incentive to let up, and it remains in search of the next vulnerable person to infect.”